Short answer: it depends on how you’re sending it. On a US landline or an office PBX that still runs on copper, add the 1 before an out-of-area fax number. On an online fax service like pay per fax, don’t. Enter the number in international format and let the service route it. On a hosted PBX or VoIP business line (an office phone system that runs over the internet instead of copper), it’s usually optional, but a few carriers still require it.
When you need the 1
On a traditional US phone line, meaning the copper phone network (PSTN) or a legacy PBX behind it, you follow the same long-distance dialing rules as a voice call. To reach a different area code, dial 1 + area code + 7-digit number. For a number in the same area code, dial just the 7 digits.
The fax number format guide covers the format in more detail, including how the same rules translate to countries whose long-distance prefix is 0.
When you don’t need it
Online fax services want the number in E.164 international format (country code first, digits run together, no spaces or dashes), without a bare leading 1:
+1 555 123 4567for a US number+44 20 7946 0000for a UK number
The service reformats the number automatically before sending, so a stray 1 on a US number is absorbed either way. From a mobile app, our send fax from phone walkthrough shows the same rule in a full send.
When it depends
Hosted PBX, VoIP business lines, and analog terminal adapters (small boxes that connect a physical fax machine to an internet phone line) fall somewhere in between. Some carriers keep the old rule of prefixing 1 for out-of-area calls; others accept a bare 10-digit number or the full +1 form. The wrong format usually returns a fast-busy tone (the rapid on-off signal that means the call didn’t connect) or a routing failure, rather than reaching a wrong number.
If a specific fax keeps failing on a hosted line, try each of the three formats in turn: with 1, without 1, and as +1XXXXXXXXXX. Note which one gets through, then save that format in the machine’s speed-dial.
How to tell which situation you’re in
A quick self-check. If you dial from a physical handset with a wall jack and no computer in the loop, that’s a traditional line and out-of-area faxes need the 1. If you send through a web page, a phone app, or an email-to-fax address, that’s an online fax service and E.164 without the 1 is right. If you have a fax machine plugged into a small adapter or an office VoIP handset, try both formats on a number you know works. The carrier’s response will tell you which one it wants.
If you’re still not sure, ask whoever installed the line. The service ticket or the PBX admin will know without you having to test-fax an unrelated number.