Passing unanswered Ring Group call to additional Ring Groups


I have set up our Asterisk/FreePBX system so that if an incoming call to a ring group is not answered within 20 seconds it gets passed on to a global Ring Group which contains a list of all Ring Groups.

The problem I have is that when the call is passed to “All Groups” and someone picks up their extension to take the call, the other extensions continue to ring. If someone else then picks up, they don’t hear anything on the line, but the remianing extensions still continue ringing.

Can anyone suggest a reason why this happens and how I might stop it?