I have asteris2billing installed so I could issue each person a pin, but this is not needed. This is just to prevent someone from hogging the phone. A cron that issues a command function could be used but then might kick people off unless they start their conversation after the cron has passed.
The msot complex thing I would like is…
Play intriduction message, after x minutes play audio to the connected user you have X minutes left, disconnect and then play another message thank you for using X
The simplest thing is just to have it disconnect after X minutes without warning.