VoIP & Contact Center Glossary: 40+ Terms Explained

A

ACD — Automatic Call Distributor

An ACD is a telephony system that receives inbound calls and routes each one to the most appropriate available agent or queue. Routing decisions are based on rules such as agent skills, availability, caller input from an IVR, or time of day. ACD is a core component of any contact center platform and operates behind the scenes every time a caller reaches a queue.

ACW — After-Call Work

ACW (also called wrap-up time) is the period immediately after a call ends during which an agent completes associated tasks — logging notes, updating the CRM, sending a follow-up email, or setting a callback. ACW is included in the calculation of Average Handle Time (AHT) and affects how quickly an agent becomes available for the next contact.

AHT — Average Handle Time

AHT is the average total duration of a customer interaction from the moment the agent answers to the end of after-call work. It is calculated as: AHT = (talk time + hold time + ACW) ÷ calls handled. AHT is one of the most widely tracked efficiency call center metrics because it directly influences staffing requirements and cost per contact.

ANI — Automatic Number Identification

ANI is a telephony feature that automatically identifies and delivers the calling party's telephone number to the receiving system. In a contact center, ANI data can be used to look up a caller's account before the call is answered, enabling screen pops in CRM systems. ANI is distinct from Caller ID: ANI operates at the network level and cannot be suppressed by the caller in the same way that consumer Caller ID can.

API — Application Programming Interface

An API is a set of protocols and definitions that allows two software systems to communicate. In VoIP and contact center contexts, APIs enable businesses to connect their phone system or contact center platform to external tools such as CRMs, ticketing systems, and analytics dashboards. Programmable voice and SMS APIs allow developers to build call flows, send messages, and trigger actions from their own applications.

ASR — Answer-Seizure Ratio

ASR is a wholesale voice quality metric that measures the percentage of call attempts that result in a completed connection, calculated as (answered calls ÷ total call attempts) × 100. A low ASR on a route indicates high call failure rates, which may be caused by poor route quality, number unreachability, or carrier filtering. ASR is commonly tracked alongside ACD and PDD in wholesale voice termination.

Auto Attendant

An auto attendant is an automated call-answering system that greets callers with a recorded message and presents a menu of options — such as "press 1 for sales, press 2 for support." Unlike a full IVR, a basic auto attendant uses fixed menus rather than dynamic call flows. Auto attendants reduce the need for a receptionist to manually answer and transfer every inbound call.

B

BPO — Business Process Outsourcing

BPO refers to the practice of contracting a third-party provider to handle specific business functions — including customer service, technical support, and outbound sales. Contact center BPO providers operate large agent teams on behalf of their clients. Cloud-based contact center platforms are frequently used in BPO environments because they allow capacity to scale without acquiring physical infrastructure.

Blended Agent

A blended agent handles both inbound and outbound contacts, switching between them based on queue demand. When inbound volume is low, the contact center platform assigns outbound dialing work; when inbound volume rises, agents are redirected to handle queued calls. Blended agent workflows require a contact center platform capable of managing both inbound ACD queuing and outbound campaign dialing simultaneously.

C

Call Blending

Call blending is the operational practice of dynamically allocating agents between inbound and outbound work based on real-time queue conditions. When inbound volumes are high, agents are pulled from outbound campaigns; when inbound demand falls, available agents are assigned to outbound lists. Effective call blending requires supervisor oversight and a platform that can coordinate both contact types in a single agent interface.

Call Routing

Call routing is the process by which an inbound call is directed to a specific destination — an agent, queue, department, voicemail, or automated flow — based on predefined rules. Routing criteria can include the number dialled (DNIS), the caller's number (ANI), time of day, IVR input, agent availability, or skill requirements. Advanced routing strategies such as skills-based routing are managed through the ACD.

CCaaS — Contact Center as a Service

CCaaS is a cloud delivery model for contact center software. Rather than purchasing on-premise hardware and software, businesses subscribe to a cloud platform that provides inbound and outbound calling, digital channels, agent management, supervisor tools, and reporting. CCaaS platforms are updated and maintained by the vendor, allowing businesses to access new capabilities without infrastructure investment.

CDR — Call Detail Record

A CDR is a data record generated for each call, capturing metadata such as the calling number, the called number, call start time, duration, and call outcome. CDRs are used for billing, compliance auditing, fraud detection, and traffic analysis. Contact center platforms typically provide CDR exports and reporting dashboards built on CDR data.

CSAT — Customer Satisfaction Score

CSAT is a survey-based customer experience metric that measures how satisfied a customer was with a specific interaction. It is typically collected immediately after a contact via a short survey — often a 1–5 or 1–10 scale. CSAT is calculated as (number of satisfied responses ÷ total responses) × 100. It provides direct feedback on individual agent interactions and is frequently tracked alongside FCR and NPS.

CTI — Computer Telephony Integration

CTI is the technology that connects a phone system to a computer application, allowing the two to exchange data and trigger actions. The most common CTI use case in contact centers is the screen pop: when an inbound call arrives, the caller's ANI is used to look up their account in the CRM, and the relevant record appears on the agent's screen before or at the moment the call is answered. CTI reduces average handle time by eliminating manual lookup steps.

D

DNIS — Dialled Number Identification Service

DNIS identifies the specific number a caller dialled, delivering that information to the receiving system. In a contact center, DNIS allows a single phone system to serve multiple inbound numbers — different product lines, campaigns, or brands — and route each call differently based on which number was called. DNIS is typically used alongside ANI data to give agents full context before answering.

DID — Direct Inward Dialling

A DID (also called a direct dial number or DDI in the UK) is a telephone number that routes directly to a specific extension, agent, or department without passing through a central receptionist or auto attendant. DIDs are provisioned via SIP trunks or VoIP providers and allow businesses to give each employee or team their own direct number while sharing underlying call capacity.

E

Erlang C

Erlang C is a mathematical formula used in contact center workforce planning to calculate the number of agents required to handle a given call volume at a target service level — typically expressed as the percentage of calls answered within a specified number of seconds. The formula accounts for call arrival rate, average handle time, and desired service level. Erlang C is the standard model used in workforce management (WFM) tools and staffing calculations.

F

FCR — First-Call Resolution

FCR measures the percentage of customer issues resolved on the first contact without requiring a callback or follow-up. It is calculated as (calls resolved on first contact ÷ total calls) × 100. FCR is considered one of the most important contact center KPIs because it correlates directly with customer satisfaction: customers who have to call back multiple times about the same issue report significantly lower satisfaction scores.

I

IVR — Interactive Voice Response

An IVR is an automated telephony system that answers inbound calls, plays recorded prompts, collects caller input via keypad or voice, and routes calls to the appropriate destination based on that input. IVR systems range from simple two-level menus to sophisticated multi-step flows that can authenticate callers, deliver self-service information, and pass contextual data to the agent who receives the call.

M

MOS — Mean Opinion Score

MOS is a numerical measure of voice call quality, originally based on subjective listener ratings and now calculated algorithmically from network metrics. Scores range from 1 (unacceptable) to 5 (excellent), with 4.0 generally considered the minimum acceptable threshold for business calls. MOS is affected by packet loss, latency, and jitter on the network path carrying VoIP traffic. Network monitoring tools use MOS to flag degraded call quality before users report complaints.

N

NPS — Net Promoter Score

NPS is a customer loyalty metric based on a single survey question: "How likely are you to recommend us to a friend or colleague?" Respondents score 0–10. Those scoring 9–10 are Promoters, 7–8 are Passives, and 0–6 are Detractors. NPS = % Promoters − % Detractors. Unlike CSAT, which measures satisfaction with a specific interaction, NPS measures overall brand sentiment and is used as a strategic customer experience indicator.

O

Omnichannel

Omnichannel in a contact center context means managing customer interactions across multiple communication channels — voice, chat, email, SMS, and social messaging — from a single unified platform. Agents see the full interaction history regardless of channel, and customers can switch channels without having to repeat themselves. Omnichannel is distinct from multichannel, where channels operate in separate systems without shared context.

P

PBX — Private Branch Exchange

A PBX is a private telephone switching system used by a business to manage internal and external calls. Traditional on-premise PBX systems require dedicated hardware installed on-site. Cloud PBX (also called hosted PBX or virtual PBX) moves that switching function to servers managed by a provider, eliminating the need for on-site equipment. Most modern business VoIP systems are cloud PBX deployments.

PDD — Post-Dial Delay

PDD is the time elapsed between the moment a call is dialled and the moment the caller hears ringing or an answer. PDD is a wholesale voice quality metric: high PDD (typically above 5–7 seconds) indicates slow routing through intermediate carrier nodes. It is tracked alongside ASR and ACD in route quality monitoring and can be a symptom of overloaded transit carriers or inefficient routing paths.

Predictive Dialer

A predictive dialer automatically dials multiple numbers simultaneously and connects answered calls to available agents, dropping unanswered calls, busy signals, and answering machines before they reach the agent. A pacing algorithm adjusts the dial rate based on agent availability and historical answer rates to minimise agent idle time. Predictive dialers are suited to high-volume outbound operations and require careful configuration to stay within regulatory call abandonment rate limits.

Progressive Dialer

A progressive dialer automatically dials the next number on a contact list when an agent becomes available, placing one call per agent at a time. Unlike a predictive dialer, it does not dial ahead of agent availability, which eliminates abandoned calls but reduces the number of live conversations per agent hour. Progressive dialing is appropriate when regulatory call abandonment limits are a primary concern or when the contact list requires individual attention per record.

Preview Dialer

A preview dialer displays the next contact record to the agent before the call is placed, giving the agent time to review account history, notes, or prior interactions. The agent then initiates the call manually or accepts the auto-dial prompt. Preview dialing has the lowest call rate of the three dialing modes but is best suited for complex accounts, sensitive collections calls, or high-value sales interactions where agent preparation matters.

PSTN — Public Switched Telephone Network

The PSTN is the global network of circuit-switched telephone infrastructure that carries traditional voice calls. It includes copper landlines, fibre trunk lines, and the interconnections between national carrier networks. VoIP calls originate and terminate as IP data but typically connect to the PSTN via SIP trunks and media gateways when calling standard phone numbers. Much of the PSTN is being migrated to IP-based infrastructure by carriers worldwide.

Q

Queue

In a contact center, a queue is a waiting line that holds inbound contacts — calls, chats, or messages — until an agent with the appropriate skills becomes available to handle them. Queue management is a core ACD function. Supervisors monitor queue depth, wait times, and service levels in real time to adjust agent assignments and prevent excessive hold times. Callers in queue may hear hold music, estimated wait times, or callback offers.

R

RTP — Real-time Transport Protocol

RTP is the network protocol used to carry audio and video data in real time across IP networks, including VoIP calls. While SIP handles call setup and teardown, RTP handles the actual media stream once the call is established. RTP packets include sequence numbers and timestamps so the receiving end can reconstruct the audio stream correctly even if packets arrive out of order. RTP is typically paired with RTCP (RTP Control Protocol) for media quality monitoring.

S

SIP — Session Initiation Protocol

SIP is the signalling protocol used to initiate, maintain, and terminate real-time communication sessions — including voice and video calls — over IP networks. SIP handles the negotiation of call parameters (codec, media format) and coordinates the exchange of media addresses between endpoints. It is the dominant protocol in VoIP and is used by IP phones, softphones, PBX systems, and SIP trunking services.

SIP Trunk

A SIP trunk is a virtual connection that links a business phone system (PBX or UCaaS platform) to the PSTN over the internet using SIP. It replaces traditional physical telephone lines and allows businesses to scale call capacity by adding channels without installing new infrastructure. SIP trunks carry both inbound and outbound calls and are provisioned by VoIP carriers or cloud communications providers.

Skills-Based Routing

Skills-based routing is an ACD strategy that matches each inbound contact to the agent whose skill profile best fits the contact's requirements. Skills are defined by the administrator — for example, language, product expertise, or account tier — and agents are tagged with their applicable skills. When a call or contact arrives, the ACD identifies the best available agent match rather than simply routing to the next available agent regardless of fit.

Softphone

A softphone is a software application that enables voice calls over IP from a computer, smartphone, or tablet rather than a physical desk phone. Softphones register to a SIP server or cloud PBX and present standard call controls — dial, hold, transfer, mute — in a desktop or mobile interface. They are widely used in remote and hybrid work environments because they require only an internet connection and a headset.

STIR/SHAKEN

STIR (Secure Telephone Identity Revisited) and SHAKEN (Signature-based Handling of Asserted information using toKENs) are a pair of standards that authenticate the caller ID of VoIP calls to combat spoofing. The originating carrier signs the call with a cryptographic attestation level (A, B, or C) indicating how confident it is that the calling number belongs to the caller. Receiving carriers and analytics platforms use this attestation to decide how to label or filter the call before it reaches the recipient.

T

TCPA — Telephone Consumer Protection Act

The TCPA is a US federal law that restricts how businesses may contact consumers by telephone, including automated dialling, pre-recorded messages, and text messages. It requires prior express consent for certain types of calls and messages, establishes do-not-call protections, and limits call times. TCPA violations can result in statutory damages per call. Businesses running outbound dialling campaigns should consult legal counsel to confirm their compliance obligations.

TTS — Text-to-Speech

TTS is a technology that converts written text into synthesised spoken audio in real time. In IVR systems and auto attendants, TTS allows prompts and dynamic information — such as account balances, appointment times, or queue positions — to be spoken aloud without requiring pre-recorded audio. Modern TTS engines use neural voice synthesis to produce natural-sounding speech across multiple languages and voice styles.

U

UCaaS — Unified Communications as a Service

UCaaS is a cloud delivery model that combines multiple business communication tools — voice calling, video conferencing, team messaging, file sharing, and presence — into a single integrated platform. UCaaS replaces a mix of on-premise PBX hardware and separate collaboration tools with a subscription service managed by the vendor. It is distinct from CCaaS, which focuses specifically on customer-facing contact center operations.

V

VoIP — Voice over Internet Protocol

VoIP is a family of technologies that transmit voice calls as digital data packets over an IP network rather than through traditional circuit-switched telephone infrastructure. Business VoIP systems replace on-premise PBX hardware with cloud-hosted services, reducing infrastructure costs and enabling features — call recording, IVR, analytics — that are difficult or expensive to implement on legacy systems.

VoIP Gateway

A VoIP gateway is a hardware or software device that converts between analogue or digital telephone signals (used on the PSTN) and VoIP (IP-based) signals. Gateways are used in hybrid telephony environments where a business is migrating from legacy PSTN infrastructure to VoIP but needs both systems to interoperate during the transition. They translate call signalling and media formats between the two environments.

W

WebRTC — Web Real-Time Communication

WebRTC is an open standard that enables real-time audio, video, and data communication directly within a web browser without plugins or additional software. VoIP and contact center providers use WebRTC to deliver browser-based softphone clients, allowing agents to handle calls from a standard browser tab rather than a separate application. WebRTC uses DTLS and SRTP for secure media transport.

Whisper Coaching

Whisper coaching is a contact center supervisor feature that allows a supervisor to speak to an agent during a live call without the customer being able to hear the supervisor. The supervisor listens to the call in real time and can deliver coaching guidance, prompts, or correction directly into the agent's ear. Whisper coaching is distinct from call barging, where the supervisor can speak to both the agent and the customer simultaneously.

Frequently Asked Questions