Remember how old speakers would let you know if you were about to get a cellphone call. It was like digital precognition haha
But this is different than the old <2g/edge phones, which wouldn't interfere unless you were about to get a call - because the tower said "where's this phone?" and your phone would max out it's tx and say "here i am!" and that's what you heard. This is probably incorrect, but based on my observations this is what occurred.
Remember the doodads you could put on your startac style phones on the antenna bit, with LEDs in them - they'd light up when you were about to get a call, as well, by design!