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Why Your Clock Radio Is All Abuzz About iPhones
blackbearnh wrote in with a story that's not really about the iPhone, but if your office speakerphones beep like mine does, read on: "If you own an iPhone, you may have noticed that it has a distinct and very annoying effect on clock radios, computer speakers, car radios, and just about anything else with a speaker. The folks at O'Reilly Media aren't immune, so they set out to discover just what is it about iPhones that makes them such bad RF citizens. The iPhones aren't the only bad apples in the cell phone basket and there's not much you can do about the problem. We're really in an interesting time in that there has never been so many high-powered personal transmitters just wandering loose in the world."
It just looks like someone has never had a GSM phone before.
Or, in other words, a 217Hz signal is amplitude modulated onto the GSM signal. Some electronic devices (like amplifiers) incidentally demodulate the 217 Hz and convert that to sound. 217Hz is well within the human audible range, thus... dutuh, dutuh, dutuh, dutuh, dutzzzzzzzz.....
(since it's a 217 hz square wave you get lots of harmonics as well)
Perhaps the AT&T cellsite is further away from your location than the T-Mobile cellsite. Hence, your phone has to "talk louder" for the AT&T cell to hear it.
No cellular provider would intentionally instruct your cellphone to emit more power than required, because it would be self-defeating. Excess transmit power just means unnecessary interference to nearby cells on the same frequency. The cellular protocols provide a means for controlling the power of a handset up and down as needed to get "just the right amount" of RF energy at the cell tower's receiver.