You could probably schedule an hourly task with ASA but that is only treating the symptom. The better question is to ask why the station is locking up. Can you provide more information on digital card firmware, phone firmware, type of gateways, location relative to the core? We have plenty of phones that get high volumes of calls and they never lock up. Also, please explain how they lock up. What do you see when you status the station or run a trace?