I have a problem sending a fax through a Cisco AS5350. The Router shows no logger and the fax prints the sheet "Communication Error (43)".
Do you see the call arriving the router?
You can try to use the "debug isdn q931" command if you have ISDN connection to the PSTN or use de "debug voip ccapi inout" (off-load hours) and check what is happening.
If you want, post the results here. Try also to explain the cen?rio. Where is the fax connected to? Interface to PSTN, etc.