Cellular traffic noise on iPhone
All phones have done this, since they were first invented - the analog ones used to be far worse. It used to be compulsory to switch them off in the studios where I worked, as the interference could actually make its way onto a recording.

It's caused by electromagnetic induction to a nearby amplified circuit - hi-fi, tv, computer speakers etc.

It was always known as 'hunting' - not that it's any real technical term, but it roughly describes what's going on. If the phone is bordering one cell &amp; hunting for the next, or if a signal comes through that data is about to be fired at it, the radios ramp up their power ready for it &amp; the beep boppidy bop beep that you hear is the handshake as it ramps up.

Turning the phone 90˚ may help, by shifting the induced magnetic field out of alignment with the circuitry that's picking it up.

The only alternatives would be to move it well away from any other electronics, or switch it to Airplane mode.

This answer gives some simple background as to how induction works; it's the same for radio circuitry as audio.