Your speedometer in the car is the only device you can rely on, as it is part of the ‘roadworthy’ vehicle.
If you were to challenge Speed Camera monitoring equipment and the calibration thereof, then you first will need to ensure your car speedometer is correct.
Then, if your speedo states 44mph and your ‘phone’ speed suggests 42mph (and you consider that to be more accurate, as its GPS triangulation based); then if your speedo shows 30mph, then your phone should be a more-accurate 28.6mph – thereby showing you to be within the MAXIMUM speed limit.