Phone Lines Not Releasing for 10 minutes

We have a problem with a customers Free PBX phone system we setup. The lines are not releasing after they hang-up for at least ten minutes.
What are some ares that I should look at first. I am new to Free PBX.

If that is POST line, check if your post line alternate the line polarity. If not, contact the phone company for that.

If you can find an very old phone with lighted keypad (not using battery or external power, just line power), plug the POST line into it then dial the POST number. When hang up the calling phone, watch the lighted keypad if the light blinks. If that does not blink, Asterisk may have problem to determine the incoming line hang up.

Some other phone system, like Nortel, detect the tone change in the incoming line. Asterisk detect the voltage change. Maybe in the future they could also detect the tone changeā€¦