See Configuring Syslog Logging in AOS - Adtran Support Community .
If no luck, at the Asterisk command prompt, type
pjsip set logger on
or
sip set debug on
according to whether Adtran entensions are pjsip or chan_sip. The SIP traffic will then appear in the regular Asterisk log.
Or, run tcpdump on the PBX and move the capture file to your PC for analysis with Wireshark.