A friend just sent me a disc from a live recording he made over the weekend using his Zoom 4 hand-held recorder. Several times during the performance there was a serious interference problem from people nearby who were using GSM cell phones (AT&T, T-Mobile, etc). Even though their phones were turned to silent they were still powered on and whenever they had an incoming signal it created a powerful series of pulses (217 Hz with a 12.5% duty cycle). CDMA cell phones (Verizon, Sprint, etc.) do not seem to exhibit this problem. Have any of you encountered this same difficulty and if so how did you deal with it. My friend was using the mics that were built-in to the Zoom 4. I don't know whether or not using external mics on the balanced XLR inputs would make any difference. I could not give any recommendation since I have not experienced this personally.