I have two lines on my PAP2T and the only thing that solved this problem rather instantly was to change the line with the problem to using port 5060 instead of 5061 (so both lines are using port 5600). Note that I am not forwarding either port in my router either.
Some of the symptoms of this problem for me were the following:
linksys, pap, dialtone, calleri, instantly, forwarding, router, caller,