I bought an AVAYA J129 to have a little play around with and see if I can setup an SIP server and connect the phone to it alongside a softphone for testing. Unfortunately after placing my IP in, the TCP port and selecting TCP I get the dreaded circle with a line through it. I am unsure of next steps to take. Phone connected fine to linphone servers.
Avaya phones are pretty much a non-starter with FreePBX/Asterisk. They require a static IP address and a lot of pain to get them registered. If you value your time, sell the phone back and get something normal like a Yealink, Grandstream, or other fully open SIP compatible phones.
While some Avaya phones support SIP, it is usually in a non-standard form designed to work with their own servers.