Delay after hangup, cellphone
I have a similar problem, not 100% sure that its due to using a cell, but every morning when I come in to work there are at least 2 ZAP PSTN trunks being held up and it usually says they've been held up for like 15-19 hrs. I always have to check throughout the day to make sure the lines aren't being held up so my customers dont get busy signals.
Welcome to the world of FXO ports
It can be any number of things Volts / ohm's / disconnect supervision / wiring
start with this
disconnect supervision
http://www.voip-info.org/wiki-Asterisk+Disconnect+Supervision

Member Since:
2008-05-19