I have run into this problem several times. I had a RAD connected to the first system ONS port. Ext 100. For no reason it would start ringing a room instead of the RAD. My cure was to change programming to wakeup with a tone and then change it back to the RAD. I think Mitel admitted to having a glitch in the software but this all happened around the time the SX50 was being discontinued and they were not interested in trying to fix it.
This system is still in service and about every 6 months I get a call that it is happening again.