When I flew with a guy a few months back in the primitively instrumented C172, his mobile phone signals had nil effect. Also I know of pilots texting family whilst on an ILS with nil effect also.
In the air is not the best time to find out what impact a mobile has on your avionics. There is some discussion bout this on an earlier thread
http://www.pprune.org/private-flying...-phones-2.html