I HIGHLY recommend getting a phone with 3G GSM (UMTS/HSPA+) that is quad-band with 850/900/1900/2100. These include the Nexus, iPhone and many VZW global phones. Reason is that many technologically advanced countries like the US, Canada (Bell/Telus/Sasktel/MTS), Japan and South Korea and Europe are decreasing the available GSM spectrum to just a sliver and reallocating it to UMTS to support data services and increased voice capacity due to the massive capacity of the CDMA based spread spectrum in WCDMA thereby if there is a-lot of users on 2G GSM your call may get dropped. AT&T has already done this in NYC and T-Mobile through their US network modernization program is re-allocating the majority of their 1900MHz spectrum to UMTS/HSPA+.

T-Mobile UMA through WiFi is a GREAT IDEA.