I have a fax line connected to an FXS voice gateway port. The caller ID from the ports to external number only shows the 4 digit number of the CUCM route pattern, not the full 10-digit DID. My provider will be blocking any calls that do not show a valid DID in the SIP invite and have flagged this number.
The config on the voice gateway:
voice-port 0/2/2 station-id name Sales Fax XXX-XXX-7725 station-id number 7725 caller-id enable