Mobile phones HAVE effect on the 737-300/400!
In a very unofficial 'test' done by the guy who checked me out on the 737 (in CAVOK conditions, I hasten to add!), he found out that if he made a phonecall, or received a phonecall, on the ILS the autopilots WILL disengage.
He performed his unofficial 'test' after hearing the 'dit dit dit' mobile sound in his headset. At the time they were flying CAT2 ILS into Copenhagen and both autopilots disengaged during the mobile sound. After landing he found out from the FA's, that one of the pax on bussness class had used his mobile during the approach.
So, receiving or making a phone call can/will disconnect autopilots.