I am assuming you mean that the trunks don't release if a caller hangs-up on the voicemail system? If so, then you will need to verify that the telco is sending a disconnect signal, and that the trunk release timer is set low enough to recognize the disconnect signal. This is typically done with a butt set or volt meter and when a caller hangs up, you should see the voltage drop to 0 for about 0.5-1.5 seconds a few seconds after the caller hangs up, if not then the telco is not sending a disconnect signal, and you should contact them to find out why.
