Had a similar problem and I had to train the night clerks to go and look at the cards in the switch and note which if any of the lights were flashing. I found a jack corroded in a room. Light on card would flash as if it were being pulsed by an old dial type phone.
I also had an SX50 where different rooms started getting calls for wakeups instead of the RAD that was on the first analog port. This has happend several times. Deprogram first port as a RAD and reprogram it fixes it every time.
Just a few ideas. Hope it helps.