There could be several possible issues. First off, what exactly happens?

Are you sure you're connecting the Axxessory Talk with the correct Mod-Tap? The one you want is the 804.2420 and is different than what is used for other applications.

What about cabling? The correct pin out is available in Appendix C of the manual. I will say that Inter-Tel specifies the use of a 4 pair cable, and while you seldom see them used, I have had connection problems in the past that went away when I made up a 4 pair connection cable and used it in place of the 3 pair.