Your fax number is the number of the phone line connected to the fax.
With a fax machine or computer running fax software, the fax number is the phone number of the attached phone line. Calling the toll-free number 1-800-444-4444 (operated by MCI) will tell you the line’s phone number.
You can also use the fax to dial a phone with caller ID (such as a mobile phone) and see what number appears on the caller id. The number that appears is the fax number the fax is using at the moment.
With an online fax service, the fax number is the number assigned by the fax provider. Log in to the service and check settings to see the fax number assigned to your account.
The fax line number is the number where it can be reached and the number that will show up on caller ID when it dials out.
If you do not update the CSID and TSID information on a fax machine when it changes fax numbers, it will print the old fax number in the header information of the faxes it sends and on confirmation pages it prints.