I would hazard a guess on one of the unlicensed bands. I would guess at 2.4GHz.
As it's an unlicensed frequency, there's very little they can do to stop the problem. Everything from microwaves and baby monitors to WiFi and Bluetooth use the 2.4GHz band. Basically anything wireless uses uses that band. The 5GHz band only got released recently.
Try turning off the WiFi and Bluetooth on your phone to see if it stops.
Also, modern components may be a lot better at picking up signals that those from 20 years ago making recent devices more sensitive to this sort of problem.