The CAA report titled " Effects of Interference from Cellular Telephones on Aircraft Avionic Equipment "
here
The term "avionics" equipment should be noted as this is exactly what was tested.
The testing involved simulation of electronic equipment, including mobile phones operating 30cm from avionics equipment. The conclusion:
-Compass froze or overshot actual magnetic bearing.
-Instability of indicators.
-Digital VOR navigation bearing display errors up to 5 degrees.
-VOR navigation To/From indicator reversal. VOR and ILS course deviation indicator errors with and without a failure flag.
-Reduced sensitivity of the ILS Localiser receiver.
-Background noise on audio outputs.
The equipment covered in testing was:
"...VHF communications, VOR Navigation, Localiser and Glide Slope equipment, together with a gyro- stabilised remote reading magnetic compass."
I wonder what effect, when held 30 cms from such equipment, a ferromagnetic material would have?