Won't stop ringing on a hangup

If someone calls in to our * system and then hangs up before the phone is answered. In many cases the phone that is ringing according to the dial plan will continue to ring even though the caller has hung up. Go to answer and there is obviously noone there.

Any thoughts on why this is happening and what needs tweaking?

Do you specify a Hangup at the end of the extension?

I am using AMP so can you tell me if this step is necessary?