I've noticed it mostly when I have the phone next to the amplification device rather than the cables.

For example, if someone puts their phone near my empeg, its speakers, or the wires, no problem. But as soon as they put their phone in a backpack in my car's trunk where the amps are, we get the noise.

The only way to prevent it would be to figure out a way to improve the RF-shielding around the amplification device that's susceptible to the interference. (RF-shielding the phone would work too, but would make it so that you can't receive calls.)