[SR-Users] PRE-PAY update timer

Hello all (again), I am looking at a 'good' method to start a call with "chunks" of time allocated. I wish the initial call to check a credit, and if credit is good, allow a small amount of time (say 30 seconds). I have enabled DIALOG and have looked at CNXCC, but am confused! When the initial 30 seconds runs out, I want to authorise another 30 seconds, etc, etc, etc. I have started to play and set a dialog timer that expires to a route, and then can 'return' allowing the call to continue - BUT was not able to reset the dialog timer to trip again after another 30 seconds. I tried to 'set_max_time' on cnxcc, but this did not seem to trip the event route upon timeout? Does anyone have a pointer to any examples? I also am unsure of how to update the timer and keep a call in play! Steve