There are 2 methods to determine your fax number:
All telephone extensions that begin with the extension number 1 have a fax number beginning with 5 and the last three digits of their extension.
x1236 is fax 5236.
All telephone extensions that begin with the extension number 2 have a fax number beginning with 4 and the last three digits of their extension.
x2555 is fax 4555
All telephone extensions that begin with the extension number 6 have a fax number beginning with 5 and the last three digits of their extension.
x6149 is fax 5149