720 Area Code: Denver, Boulder and Colorado's First Overlay

What 720 covers

720 has no boundary of its own; it covers whatever 303 covers. When the Colorado Public Utilities Commission (PUC) announced 983, it described the shared area as the greater Denver region and listed fourteen places in it:

Aurora · Boulder · Brighton · Castle Rock · Columbine · Englewood · Erie · Highlands Ranch · Lakewood · Littleton · Longmont · Thornton · Westminster · Wheat Ridge — and Denver itself.

That list isn't exhaustive, and neither the PUC nor NANPA defines the area by county. The more exact edge comes from the rate centers — the local exchanges numbers are issued in — shown on NANPA's overlay map in Planning Letter 563. They stretch further than "Denver" suggests:

• East onto the plains: Bennett, Strasburg, Byers, Deer Trail and Roggen.
• North-east: Fort Lupton, Hudson and Keenesburg.
• Into the foothills: Allenspark-Lyons, Georgetown and Deckers.

So a 720 number isn't proof of a Denver street address, and it never was proof of anything after the line moved. A mobile number keeps its code wherever its owner goes.

303, 720 or 983: which one a new line gets

The three codes are not issued evenly. The PUC approved 983 on 21 May 2021 (Decision C21-0310), and the conditions in NANPA's Planning Letter 563 say 983 prefixes can be assigned only after every available 303 and 720 prefix has been used. 983 went into service on 17 June 2022, but that date only made the code usable — it didn't mean new Denver phones started receiving it.

Why add 983 at all? In 2020 the combined 303/720 area was projected to run out in the second quarter of 2023, and the industry wanted a new code in place about six months before that. NANPA's April 2026 exhaust analysis now puts the three-code area's exhaust in the second quarter of 2069 — forecasts have moved a long way since 2020, so read any single projection with its date attached.

Denver dialed ten digits first

720 changed how the Denver area dials, and it did so years ahead of the rest of Colorado. NANPA's 1998 planning letter for the 720 overlay records the sequence:

1. Jul 1997

   303 declared in jeopardy

   With prefixes running out, a lottery rationed new prefixes to nine a month from September 1997.

2. Feb 1998

   Ten digits allowed

   From 1 February 1998, local calls could be dialed with seven or ten digits.

3. Jun 1998

   720 goes live

   The overlay entered service on 1 June 1998, at the end of the four-month permissive period.

4. Oct 2021

   The rest of Colorado follows

   719 and 970 moved to mandatory ten-digit dialing on 24 October 2021 to make room for the 988 crisis line.

The Colorado State Library notes that 720 was the state's first overlay, which left metro Denver as the only part of Colorado dialing ten digits locally until 2021. One date is still disputed: some sources give 1 September 1998 for 720. NANPA's letter and its area code database both give 1 June; the September date may mark a later stage of the dialing change, but we haven't found a primary source that says so.

Today a local call anywhere in the area is ten digits; carriers may also accept 1 + ten digits. Long distance is 1 + ten digits, and 911 and the other three-digit services still work. If you're updating a phone system or dial plan for an overlay area, our guide to area codes and call routing covers the parts that tend to break.

Mountain Time, with daylight saving

The 720 area is in the Mountain time zone, defined in federal rules at 49 CFR 71.7. Colorado observes daylight saving time under the Uniform Time Act: clocks move forward at 2 a.m. on the second Sunday in March and back on the first Sunday in November.

This is where "Mountain Time" gets confusing. Arizona is also in the Mountain zone but doesn't change its clocks, so for most of the year a Denver phone and a Phoenix phone show different times — the 480 area code guide explains Arizona's rule.

Is 720 a country code?

No. Searches for "720 country code" are common, but 720 is a US area code. The United States is part of the North American Numbering Plan, which uses country code 1. To call a 720 number from outside North America, dial your international exit code, then 1, then 720 and the seven-digit number — written +1 720 XXX XXXX.

Colorado's other area codes

More US codes, grouped by time zone, are in our area code guides.

Questions

Sources

1. NANPA — Planning Letter NANP-117: overlay of 303 with 720 (24 March 1998; archived copy)
2. NANPA — Planning Letter 563: 983 and 303/720 all-services overlay, with rate center map (29 July 2021; archived copy)
3. Colorado PUC — New 983 area code is coming to the 303/720 region (17 May 2022)
4. Colorado PUC — Written comment on the area code overlay plan (13 November 2020)
5. Colorado PUC — Colorado's new area code 748: FAQ
6. NANPA — Area code (NPA) database, file dated 16 September 2026
7. NANPA — 2026-1 NPA exhaust analysis (April 2026)
8. Colorado State Library — Ten-digit dialing for area codes 719 and 970 (April 2021)
9. 49 CFR 71.7 — Boundary line between central and mountain zones
10. 15 U.S.C. 260a — Daylight saving time
11. NANPA — About the North American Numbering Plan