Last updated · Written for people who have just been asked for a fax number and do not have one.
A fax number is an ordinary telephone number that is answered by a fax machine or an online fax service instead of by a person. In the US it has the same ten digits as any phone number — an area code plus a seven-digit local number. When someone dials it from another fax, the two ends exchange tones and send a page across as an image. Nothing in the digits marks it as a fax number; only what answers is different.
How is a fax number different from a phone number?
It is not, technically. A fax number is allocated from the same North American Numbering Plan as every other US number, it can be a local number tied to an area code or a toll-free number in the 800, 888, 877, 866, 855, 844 or 833 range, and it can be moved between providers like any other number. The difference is entirely in what picks up.
| Phone number | Fax number | |
|---|---|---|
| Format | Area code + 7 digits | Identical |
| What answers | A person or voicemail | A fax machine or an online fax service, automatically |
| What is exchanged | Speech | A page image, encoded as tones over the call (ITU-T T.30) |
| What you get afterwards | Nothing on paper | A printed page, or a PDF, plus a transmission report that records whether it arrived |
| Can it be both? | Only with a switch or a distinctive-ring setup. In practice a business that faxes at all keeps a separate number, which is why a form asks for “Phone” and “Fax” on two different lines. | |
What actually happens when someone faxes you?
The sending machine dials your number. Your end answers and emits a tone that says “I am a fax.” The two negotiate a speed and a page size, and the sender transmits each page as a compressed black-and-white image under the ITU-T T.4 and T.30 standards. At the end, the receiving end confirms what it got, and the sender prints or records a confirmation. That confirmation — the reason people still fax — is a record that a specific document reached a specific number at a specific time.
If your number is on an online fax service rather than a machine, the service answers the call for you and emails you the result as a PDF. There is no paper, no dial tone in your office and nothing to run out of toner. That is the whole of receiving a fax online.
Do I need a fax number, or just a way to send?
These are two different needs and it is worth being clear which one you have.
- You only need to send. You have a form to get to an agency, a landlord or an insurer, and they gave you their fax number. You do not need a number of your own at all — sending a fax uses theirs. A single send costs $2.99 on usfax.com Pay Per Fax.
- Someone has to fax you back. Then you need a number of your own, because there has to be something on the other end to answer. That is what a subscription buys: usfax.com Solo is $19 a month ($15 billed annually) and includes one dedicated US fax number and 300 pages.
Who still asks for a fax number?
More organisations than most new business owners expect, and the pattern is consistent: fax is usually one of two or three published channels, and it is often the fast one. Verified examples, all read in September 2026:
- IRS Form 8655 (Reporting Agent Authorization) — fax to 855-214-7523 or mail. There is no online channel for it.
- IRS Form 2553, the S-corporation election — mail or fax to 855-887-7734 (Kansas City) or 855-214-7520 (Ogden).
- Oregon Health Authority Medicaid provider enrollment — “Fax your application to 503-378-3074” when the portal cannot be used.
- Texas Medicaid (TMHP) provider enrollment — fax 512-514-4214, published next to the portal and the mailing address.
- C2C Innovative Solutions, the CMS contractor for Medicare Level-2 appeals — fax 904-539-4081, one of only two secure electronic routes.
Every number above is published by the agency itself; the links are in the Sources section at the foot of this page. None of these processes is fax-only except Form 8655, and we say so rather than overstating it.