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Colorado · United States

Area code 303: where it is and who calls from it

  • MT (UTC−7/−6)
  • Overlays: 720, 983

Reviewed by Sujan ThapaliyaLast updated

What is area code 303?

Area code 303 serves Denver, Boulder, Aurora, Lakewood in Colorado, United States. It is in the MT (UTC−7/−6) time zone, and it shares its territory with the overlay codes 720 and 983.
Where area code 303 sits in a North American number: the three-digit area code, then the three-digit exchange code, then the four-digit line number, all under country code 1. The diagram also shows the two validity rules — an area code never starts with 0 or 1, and a repeated middle digit marks a service code such as 800 for toll-free rather than a place.
The 1 you dial before a ten-digit number is a long-distance prefix, not part of the number itself.

Area code 303

Type
Geographic area code
State / province
Colorado
Country
United States
Main cities
Denver, Boulder, Aurora, Lakewood
Time zone
MT (UTC−7/−6)
Overlay codes
720, 983
Same territory, different code.
In service since
1947

Step by step

How to call a 303 number

The same ten digits, dialled three different ways depending on where you are calling from.

  1. 1

    Within the same area code

    Many carriers still allow seven digits inside 303, but overlays have made ten-digit dialling the safe default — 303 overlaps with 720 and 983, so seven digits is ambiguous. Dial 303 followed by the seven-digit number.
  2. 2

    From elsewhere in the US or Canada

    Dial 1, then 303, then the seven-digit number — eleven digits in total. The leading 1 is the North American long-distance prefix, not part of the number.
  3. 3

    From outside North America

    Dial your country's exit code, then 1 for the North American Numbering Plan, then 303, then the seven-digit number. On a mobile, type + instead of the exit code: +1 303 followed by the number works from any country.

Background

About 303

303 is metropolitan Denver and the Front Range.

Unwanted calls

Why you are getting 303 calls

The reason an unfamiliar 303 number keeps calling is usually neighbour spoofing: automated systems fake a caller ID in the recipient’s own area code because a local-looking number is answered far more often than an unfamiliar one. The number displayed is frequently not the number placing the call, and may belong to someone with no idea it is being used.

What actually helps:

  • Let unknown numbers go to voicemail. Automated diallers rarely leave a message; a real caller will.
  • Do not press any key to be removed. On an automated call, a keypress confirms a live human and increases the volume you get.
  • Register on the national Do Not Call list and report unwanted calls. It does not stop fraud, but it does constrain legitimate telemarketers.
  • Turn on your carrier’s call-filtering service. Filtering now uses STIR/SHAKEN attestation, which is why properly signed business calls get through and spoofed ones increasingly do not.

If it is your 303 business number being labelled as spam, the cause is different and fixable: unregistered caller ID name, low attestation, or a volume pattern carriers read as automated. Owning your numbers, registering CNAM and signing at Attestation A is what resolves it.

If you are calling into 303 rather than receiving from it, the relevant reading is local presence dialing — the same signals that decide whether a call from this code gets answered decide whether yours does.

Frequently asked questions

Where is area code 303?
Area code 303 covers Denver, Boulder, Aurora, Lakewood in Colorado, United States.
Is a call from 303 a scam?
Not inherently. 303 is a legitimate area code serving real people and businesses in Colorado. But spam callers frequently spoof caller ID to display a number in your own area code, a technique called neighbour spoofing. If you do not recognise the number, let it go to voicemail; a genuine caller will leave a message.
What time zone is 303 in?
303 is in the MT (UTC−7/−6) time zone. Check the local time before calling back so you are inside normal business hours.
Can I get a 303 phone number?
Yes. Local numbers in 303 can be provisioned in minutes and used for calls, texts, and as your outbound caller ID. A number in the area code you are calling is answered noticeably more often than an out-of-state one.
What is the difference between 303 and 720 or 983?
There is no geographic difference. 720 and 983 are overlays covering exactly the same territory as 303. Overlays were introduced when the original code ran out of numbers, which is why both require ten-digit dialling even for local calls.

Sources

  1. North American Numbering Plan Administrator — NANPAArea code assignments, relief planning, and which codes are in service across the North American Numbering Plan.
  2. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

Get a 303 number for your business

A local 303 number, owned by you, with your business name on the caller ID and callbacks routed straight to your team. People answer local numbers, and they answer branded ones far more often still.

  • Branded caller ID (CNAM)
  • STIR/SHAKEN Attestation A
  • No subscription
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