Instant check
This checks the number's format. To know for sure a fax will connect, you'd need to send one.
The check happens right here in your browser. The number never leaves your device.
What does it mean to validate a fax number?
A fax number is an ordinary phone number, so the same rules that apply to any phone number apply here. Validation just answers a few different questions:
- Determining whether a given number could be correct, for its country and type (landline, mobile, special service number).
- Determining whether a number actually does connect to a live endpoint.
- Determining whether a live number actually leads to an expected person, business, or entity.
- Determining whether a number is capable of responding with the service you expected: voice, text, fax, data, etc.
Is the fax number in the right format?
The instant check above answers this for a single number: it parses the digits with Google’s libphonenumber and tells you whether the format is plausible for its country. If you want to understand what it’s doing, or check formats yourself:
- Read the National conventions for writing telephone numbers article on Wikipedia to see what a number in your destination country generally looks like.
- For developers: use libphonenumber directly.
- For developers #2: avoid trying to do this with regex.
Is the number live?
- Make a test call from your office or mobile phone to see if you get an answer.
- If your available phones are blocked from the destination you wish to test, make that test call with a VoIP calling app like Google Voice.
Does the number belong to who you expect?
- Look up the number in an official phone directory to find out if the number and owner match up to your expectations.
- Look up the number on social media sites, to make sure it’s “valid” according to the public.
Is it really a fax line?
This is the one question a phone-number check can’t answer, and for most people it’s the point of the exercise.
The audible test is the quickest one. Call the number from a normal phone and listen. A dedicated fax line answers with a steady high-pitched tone (the CED tone, roughly 2.6 to 4 seconds at 2100 Hz) and then breaks into modulation that sounds like a warbling screech. That’s the machine telling you it’s ready to receive.
A voice answer is not proof that it isn’t a fax line, though. Shared voice-and-fax lines and fax-to-email services like eFax or RingCentral usually pick up with a human greeting or a menu, and only switch into fax mode when they hear the CNG calling tone from a sending machine. They need the caller to sound like a fax before they answer like one.
The sure test is to send a one-page fax and see if it delivers. That’s the only positive proof you have a working fax line at the other end.
Depending on how accurate you need your validation to be, you can use any or all of these methods.
For entertaining reading, check out a bunch of falsehoods people believe about phone numbers.
Testing your fax machine’s sending ability
What if you want to test if your fax machine is sending correctly? We have a solution for you. Visit the online fax tester on our sister site at https://faxbeep.com/. Here, you’ll be provided with a number to which you can send a fax. Once a fax is received at that number, it will get displayed on the fax-tester page. It’s a simple and effective way to confirm your fax machine’s sending ability.