Unfortunately, you guys with newer iPhones will still get the GSM buzz as soon as the phone loses 3G and falls back into Edge data mode.
Boring facts:
It has nothing to do with the radio carrier frequency (800MHz, 1800MHz, etc) but is related to the 'packet transmission frequency'. That is, it only sends a burst of data every so often, and it just happens to do that at regular intervals about 218 times a second. It's that 218Hz tone that is audible.
There are two useful implications of this:
One is that it requires an active element to be audible.
Bare headphones -> no sound. Headphones plugged into a radio or amplifier -> buzz. This is due to the headphone wire picking up the radio signal as an antenna, conducting it to the active elements in the amplifier, the amplifier converting it in a way similar to an AM radio, and sending it back to the headphones as audio buzz.
The second is that it is possible to design electronics that reject the carrier frequency OR the audio frequency, or both.
The carrier frequency can be very difficult to filter, depending on how close it is to the active electronics. In this specific instance, it sounds like the phone is inducing the ANR circuit in the headset to buzz. I have also seen a PS Engineering intercom pick up the buzz with regular headsets, when the headset cord runs by my pocket.
There are a few things I'd recommend in your case:
Move the phone away from your headset wiring
Turn the phone off (obviously)
If those aren't options, you can experiment with snap-on ferrite EMI supressors:
http://mouser.com/Passive-Component...-RFI-Suppressors-Ferrites/_/N-194jj?P=1yzv8sh
That link should show you only the snap-on type. Try to find one that matches your cable diameter (the ID or inner dimension of the ferrite should be close to the cable you're snapping it on).
Put them between where interference might be getting in (long lengths of wire) and active elements. So, something I might start with, is headset-ferrite-cable-ferrite-jack, with the ferrite placed close to the active element it is protecting.
There is no guarantee to this approach, but it might do what you want.