If I didn't know any better - it sounds like the sip provider is sending a keep-alive message for the first call in the form of a invite
you would need access to the command line of the 908 to do a debug sip stack messages
make a test call for at least 15 minutes - you will probably see you call setup again as a new invite
from there - in the sip trunk - is some options for handling sip keep-alives
We are not aware of any issues like this that would be caused by the ADTRAN unit. The person that has access to and administers the ADTRAN unit would need to get debug output of this problem occurring. A packet capture may also be necessary.
debug sip stack message
debug voice verbose
debug isdn L2-formatted
I'm going to put your ticket as "assumed answered", but feel free to respond with any updates or follow up questions you may have.