What is a fax T1_TIMEOUT?
Sometimes when a fax fails, it may be due to what's referred to as a T1_TIMEOUT (as reported on the Asterisk CLI or in a debug log). What this means is that there was a problem in Phase A, which is where both ends of the fax dialog attempt to establish communication.
If you see that a fax failed due to T1_TIMEOUT, there are some steps you can take in an attempt to alleviate the problem.
For faxes over SIP (T.38):
In order for T.38 faxes to work, they will need to utilize the UDPTL ports as defined in the /etc/asterisk/udptl.conf file. By default these are 4000-4999, but it would be a good idea to check the udptl.conf file to ensure which ports are being defined.
Make sure that the firewall/router is allowing traffic through the UDPTL ports as defined in the udptl.conf file.
For faxes over DAHDI channels:
Line quality can dramatically impact faxes; ensure that the lines are clean and free from noise or interference.
If there is a problem with the line quality, you may need to replace the cabling or contact your telco about it.