GSM (T-Mobile / AT&T Wireless) phones in general will cause interference in systems that aren't shielded well from them, as will iDEN/TDMA (Sprint). CDMA phones (Verizon / Sprint) don't have the issue as prevalently, nor do 3G AT&T Wirelesss / 3G T-Mobile phones.

Back to the topic, the phones would causing buzzing, rather than beeping...and it would be random--unless on a same path every day, and the tower was chatty with the phone. (During low signal and/or tower hand-offs.)

Any way you can record the sound so we can hear it?

Joel