What Is a Fax Number? How It Works and How to Find Yours on a Printer

Quick answer: A fax number is the phone number of a line connected to a fax machine, a fax-capable printer or an online fax service. In the US it's a regular 10-digit number, like (555) 555-0142. When someone dials it, the machine on the other end answers with fax tones instead of a voice and prints the document it receives. A printer doesn't come with a fax number of its own: its fax number is the number of the phone line plugged into its LINE port.

Published November 12, 2026 · By Lauren Elward, Castle Ink

Key Takeaways

• A fax number looks exactly like a phone number: area code plus seven digits in the US, or +1 and ten digits internationally.
• Your printer's fax number is the number of the phone line it's plugged into. The printer only stores and displays it.
• To find it, check the printer's fax header setting (HP Fax Header, Brother Station ID, Epson Your Phone Number, Canon Unit TEL), print a fax report, call out from the line, or ask your phone company.
• A printer with no phone line connected has no fax number. Wi-Fi alone can't send or receive a fax; an online fax service can give you a number.
• Calling a fax number from a phone gets you a screech or beeps. It's harmless, and it's a quick way to confirm a number is a fax line.
• US rules require faxes to identify the sender, including a phone number for the sending machine. That's why setting your fax header matters.

What Is a Fax Number?

A fax number is a telephone number that is answered by a fax machine instead of a person. It tells the sender's fax machine which line to call. Once the call connects, the two machines exchange tones to agree on speed and resolution, then send the scanned page as a stream of audio signals that the receiving machine turns back into a printed page.

The handshake is standardized. According to the ITU T.30 standard, as summarized on Wikipedia's fax article, the calling machine sends a "Calling Tone (CNG) at 1100 Hz," and the answering machine replies with a "Called Station Identification (CED)" tone before the machines negotiate settings and send the image. Those are the beeps and screeches you hear if you listen in.

A fax number can belong to:

• A dedicated fax line that only a fax machine or printer answers. This is common in medical, legal and government offices.
• A shared line that rings a phone and a fax printer, with the printer set to detect fax tones (often called Fax/Tel or Manual mode).
• An online fax service that assigns you a number and delivers incoming faxes to your email or an app.

What Does a Fax Number Look Like?

A US fax number looks identical to a US phone number: a three-digit area code and a seven-digit number. On letterhead, business cards and forms, it's usually labeled Fax or F so people don't call it by mistake.

Fax numbers are usually 10 digits in the US. International numbers are longer and start with a country code; your printer may need an international prefix (011 from the US) instead of the plus sign.

Is a Fax Number the Same as a Phone Number?

A fax number is a phone number. The difference is what answers it: a fax machine instead of a person. Technically, any landline number can work as a fax number once a fax machine or fax printer is plugged into that line and set to answer.

That means a single line can be your phone number and your fax number at the same time. Fax printers handle this with receive modes. "Fax only" answers every call as a fax. "Fax/Tel" or "Manual" lets you pick up voice calls and hands fax calls to the printer. Our guide to faxing from a printer walks through each receive mode.

How Do I Find the Fax Number on My Printer?

Check the printer's fax header or Station ID setting, which stores the number shown at the top of every fax you send. If it's blank or out of date, find the real number by calling out from the line or asking your phone company. The printer itself never assigns a number; it only remembers the one someone typed in.

1. Check the fax header setting

Every fax-capable printer has a screen where you enter your name and fax number. That text is printed in the margin of every page you send. Menu names vary by brand and model:

HP's support team puts it simply in its community forum: the fax number "is typically the phone number that the printer is connected to." If the field is filled in, that's your number, as long as nobody has moved the printer to a different line since it was set up.

2. Print a fax setup or confirmation report

Most fax printers can print a fax configuration, setup or settings report from the Fax > Reports menu (or Tools > Reports). It lists the stored header name and number. A transmission confirmation report from a recent fax shows the same information. And if you've received a fax from this printer, look at the top margin: the sender's number is printed there.

3. Call out from the line

This is the most reliable method because it tests the actual line, not the setting. Unplug the phone cord from the printer's LINE port, plug in a regular corded phone, and call your cell phone. Caller ID shows the line's number. (If the line has caller ID blocked, call a friend and ask them to read it back.) Plug the cord back into LINE when you're done.

4. Ask your phone company or check your bill

Your phone bill or online account lists every number on the account, including a separate fax line. If you're in an office, the IT or telecom person who manages the phone system can tell you which number rings the fax port, and whether you need to dial 9 first.

What If My Printer Doesn't Have a Fax Number?

If no phone line is plugged into the printer's LINE port, the printer has no fax number and can't receive faxes. This is common at home, where many people have dropped landlines. Your options:

• Connect a phone line. A basic landline works best. Cable and fiber phone service (VoIP) often works but can fail on long faxes; lowering the printer's fax speed usually helps. Wikipedia notes that fax over VoIP depends on the provider supporting T.38, or on clean audio pass-through, which "is sensitive to packet loss and jitter."
• Get an online fax number. An online fax service assigns you a number and emails you incoming faxes. You can still scan on your printer and send the file.
• Use a store's fax service for the occasional one-off.

Our guide to faxing without a fax machine compares these options.

Can You Call a Fax Number?

Yes, you can dial a fax number from any phone, but a fax machine will answer with a high-pitched tone or a series of beeps, and no one will hear you. It doesn't hurt anything. Hang up after a few seconds. Calling is actually a handy way to confirm that a number on a form really is a fax line before you send something.

If you reach a person instead, the number is either a voice line or a shared line set to Fax/Tel mode. Ask them to switch to fax (or press Start) when you send.

Can You Email or Text a Fax Number?

Not directly. A fax number is answered by a fax machine, so a regular email or text message has nowhere to go. You can send to a fax number from your email or phone through an email-to-fax or online fax service: you attach a PDF, address it the way the service tells you (often the fax number at the service's domain), and the service dials the number and sends the pages for you.

You can also scan a document on your all-in-one printer and send the file through such a service. See how to scan from your printer.

Why Does My Fax Need a Header?

US federal rules require it. The FCC's rule at 47 CFR 68.318(d) says anyone sending a fax must mark it "in a margin at the top or bottom of each transmitted page" (or the first page) with the date and time, the sender's name, and a telephone number for the sending machine or sender. Brother's manual points out the same Telephone Consumer Protection Act requirement on its Station ID page, and some US printer models won't send a fax until the header is filled in.

Setting your header also helps recipients, especially doctors' offices and pharmacies, know who sent the fax and where to send a reply.

Frequently Asked Questions

Keep Your Fax Printer Supplied

Incoming faxes print automatically, so a fax machine that's out of toner or film can silently store faxes in memory, or miss them. Castle Ink stocks compatible fax supplies, and orders placed by 4 PM ET Monday–Friday ship the same day:

• Brother laser MFCs (MFC-L2690DW, MFC-L2710DW, MFC-L2717DW, MFC-L2730DW, MFC-L2750DW): compatible Brother TN760 high-yield toner, plus the rest of the TN760, TN730 and DR730 drum family.
• Brother plain-paper fax machines: PC-401 fax ribbon cartridge (FAX-275, FAX-560, FAX-565, FAX-580MC, MFC-660MC), PC-501 fax ribbon cartridge (FAX-575, FAX-878) and PC-402RF refill rolls.
• Panasonic laser fax machines: KX-FA83 toner (KX-FL511, KX-FL541, KX-FL611) and more in Panasonic fax toner.
• Also: all fax ribbons and thermal transfer film, Sharp fax toner, and HP ink for OfficeJet fax printers.

Related guides: How to fax from a printer · How to fax without a fax machine · What is a document feeder (ADF)?

Sources

• Legal Information Institute (Cornell): 47 CFR § 68.318, Additional limitations (fax identification, paragraph d)
• Brother: Set Your Station ID (MFC-L2750DW online user's guide)
• Epson: Fax header and phone number setup (WorkForce WF-2830 FAQ)
• Canon: Specifying the Basic Settings for Faxing (User information settings)
• HP Support Community: How do I find my printer's fax number?
• Wikipedia: Fax (T.30 handshake, CNG/CED tones, fax over IP)