This isn't very scientific, is it ?. I find it hard to believe that a
phone in standby mode could affect what is after all a magnetic sensor.
Avionics kit is stringently tested for rf susceptability and sensitive
signal cables are screened. The only way that there could possibly be an
effect is via rf into one of the antennas, but a phone on standby and
not transmitting ?. Convenient to blame the device, when it could be for
any number of other reasons.
A bit more scientific study might not be such a bad idea, with various
phones and devices, under all operating conditions...