[mctavjb9]

Almost all GSM phones sold in the past 4-5 years are quad-band phones-- 850 MHz/1900 MHz (US, Canada, a few other places), and 900 MHz/1800 MHz (Europe, Asia, Africa).

An iPhone in the US, for instance, connects to AT&T for voice calls on the 850 or 1900 MHz bands, depending on what chunk of spectrum the carrier owns in the caller's particular location. If Joe Caller got on a plane to Europe, the phone might roam onto a GSM900 network. But in the US, part of the 900 MHz European GSM band happens to be allocated to ISM.

What people experimenting with OpenBTS in the US tend to do is test with old 900/1800 MHz-only phones that can be procured for a pittance on eBay. That way, they're guaranteed not to connect to AT&T or T-Mobile.

As a sidenote, before the GSM carriers started upgrading to 3G networks, most GSM phones didn't actually have GPS chips. Sprint & Verizon (CDMA) phones did, because GPS timing information is a crucial part of the standard (see http://alumni.cs.ucr.edu/~saha/stuff/cdma_gps.htm for gory details).