720 Area Code: Denver's Second Layer

The 720 area code is the Denver metro area — Denver, Boulder, Aurora, Lakewood, and the Front Range suburbs — serving the exact same ground as 303 since June 1998, when it became one of the first overlay codes in the United States.

Denver is best read the way geologists read a road cut: three layers of numbering laid down over 75 years. 303 is the bedrock, 720 is the boom-era stratum, and 983 is the thin new deposit on top. The code on an incoming call tells you roughly when the line was born — and almost nothing else.

One metro, three vintages

All three codes cover the identical footprint: Denver, Boulder, Aurora, Lakewood, Littleton, Longmont, Broomfield, and Castle Rock, per the Colorado Public Utilities Commission’s overlay filings. There is no geographic boundary between them — a 983 number in Boulder and a 303 number in Boulder are neighbors on the same switch.

What separates the codes is age. Because overlays never reassign existing numbers, each layer preserves the era that created it, the way sediment preserves a climate. That makes the Denver stack unusually legible:

Code	Laid down	What it signals
303	1947	Original code; lines and businesses that predate the overlay era
720	1998	The tech-and-migration boom; many mobile-era numbers
983	2022	The newest allocations — recent movers, new businesses, fresh VoIP lines

The bedrock: 303 and the state it once held

303 is a 1947 original, and it initially covered all of Colorado. It held the whole state for four decades before growth forced two splits: 719 took Colorado Springs and Pueblo in March 1988, and 970 took Fort Collins, Grand Junction, Vail, and Aspen in April 1995.

Each split shrank 303 down toward its core — metro Denver — which is why a 303 number still reads as old-Denver shorthand locally. Bumper stickers and business names trade on it. But the pool kept depleting, and the region needed capacity without another disruptive split.

The boom layer: why 720 arrived in 1998

By the late 1990s the Front Range held most of Colorado’s landlines, cell phones, fax machines, and pagers, and even a shrunken 303 was headed for exhaustion. The fix was an overlay: area code 720 entered service in June 1998, stacked directly on top of 303’s territory.

That made Denver an early adopter. Only a handful of overlays existed anywhere — New York’s 917 pioneered the model in 1992, Maryland’s 240 followed in 1997 — so 720 put Denver among the first US metros to run two all-purpose codes over one map, complete with the ten-digit local dialing overlays require. Our area code guide covers why the overlay model went from novelty to national standard.

The timing is why 720 reads post-boom: most new prefixes assigned during Denver’s tech-and-migration surge drew from it, alongside 303’s remaining inventory.

The new deposit: 983

Two codes bought about 24 years. On May 21, 2021, the Colorado PUC approved a third code for the complex, and 983 went into service on June 17, 2022 (NANPA Planning Letter 563). A 983 number is the freshest layer — a line activated in roughly the last four years, exactly as local as its older siblings.

Colorado’s numbering pressure hasn’t stopped at the metro line, either: the 970 region to the north and west gained its own overlay, 748, in July 2025. Growth-market metros across the West carry the same stratigraphy — Las Vegas runs a similar stack under 702.

What a 720 display actually tells you

The vintage signal is real, but it is context, not evidence. A 720 or 983 display doesn’t prove the caller is in Colorado — numbers port when people move, VoIP dials from any network, and caller ID spoofing can draw any code on your screen. Neighbor spoofing deliberately picks digits that match yours, so a familiar-looking 720 prefix deserves no extra trust.

The habit that actually screens calls: never act on an inbound caller’s claims. Hang up, find the organization’s published number on your own, and dial that instead. That routine defeats spoofed banks, spoofed utilities, and spoofed “Xcel is disconnecting you today” calls alike.

Holding a Front Range number from anywhere

For businesses selling into the Front Range, a metro-Denver caller ID measurably outperforms an out-of-state one — the mechanics are covered in our local presence dialing guide. Numbers are provisioned by rate center, so you can hold Denver-metro DIDs whether your office is in Aurora or Atlanta.

SIPNEX carries Denver-metro DID inventory across 303, 720, and 983, trunks it into whatever phone system you run, and stamps outbound traffic with A-level attestation — extensions from $6.99/mo for teams that want the full phone system behind the number.

Frequently asked questions

Is 720 a Denver area code?

Yes. 720 has served the Denver metro area since June 1998 as an overlay on 303 — the same territory, including Boulder, Aurora, Lakewood, Littleton, Longmont, Broomfield, and Castle Rock. A third code, 983, joined the same footprint in 2022, so one metro now carries three interchangeable codes.

What is the difference between a 303 and a 720 number?

Age, nothing else. Both cover the identical Denver-metro footprint, dial the same, and cost the same. 303 is the original 1947 code, so its numbers skew toward older lines and long-established businesses; 720 numbers were issued from 1998 onward. Locals treat 303 as heritage shorthand, but carriers treat the two as one pool.

Why do new Denver numbers start with 983 instead of 720?

Because the newest layer takes the new assignments. The Colorado PUC approved 983 in May 2021 and it entered service on June 17, 2022; as 303 and 720 prefixes deplete, carriers increasingly draw new numbers from 983. It is every bit as local — just the youngest stratum in the stack.

Do 720 numbers cover Boulder and Aurora?

Yes. The 303/720/983 overlay complex covers the whole Front Range core — Denver, Boulder, Aurora, Lakewood, and the surrounding suburbs — as one service area. Fort Collins and the mountain towns are outside it, on 970 (now overlaid by 748). Businesses can provision numbers in specific rate centers like Boulder or Aurora directly.

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SIPNEX is an FCC-licensed carrier providing Denver-metro DIDs and local numbers across US area codes, toll-free numbers as a registered RespOrg, and dialer-grade SIP trunking — every call signed with our own STIR/SHAKEN certificate. Call (833) 665-2220, talk to an operator, or see rates.