[Help] Hold timeout and ring back

I have just about put the finishing touches on my first asterisk server, but there is one thing I have a question about:

We have an old key system that Asterisk is replacing. When a caller is put on hold, and left there for too long, the phone which put the caller on hold will begin to ring again. This way, someone who is on hold is never forgotten.

How can I emulate this behavior in Asterisk.

I am running 1.2 checked out of svn on Friday, March 9th.

Thank you in advance!