There is a logical explanation for this and after thinking about it I have a theory. Cell phones work on different bands, but you gotta remember that a cell phone will produce a lot of noise that is not on it's band. It's the same with a tv or a blender. It's going to make noise, even though it wasn't designed to transmit, how much noise depends on the device and how much shielding was put into it.

I would say that past a couple of feet a cell phone isn't going to make enough noise to matter, but when the phone is close to the rx or tx, then you could have interference. How much interference completely depends on the shielding in the phone or rx/tx. If you have a well engineered phone and tx there shouldn't be a problem, but if you have a poorly engineered phone and tx then interference is certainly possible.

My current phone doesn't interfere with the radio in my truck, but my old phone broadcasted so much noise that you could hear the toaster buzz when it was about to ring.

So I think it all depends on the phone and radio.