You can quite often hear a phone in the aircraft on either the radio or intercom and it is identified by a rapid ticking noise when it sends packets of data. It can also go out on your transmission and be picked up by ATC. I have been told by ATC to tell my passenger to switch their phone off.
The bursts of data are more likely to be picked up by audio systems that rectify the RF, than any instrumentation.
Place you mobile near a BT phone and you will probably hear a similar noise.