rick,
It sounds as though you are not giving a positive disconnect from the POTS line and it thinks the call is still ongoing.
I would try to perform the Loop Loss and Hybrid Balance tests on all of your POTS lines and see if that fixes the problem; do each test 3 times.
If it continues I would have a Buttset on each line to see which POTS lines are having the problem and then put a positive disconnect device on those.
Thanks,
TE