All but a small handful of countries use GSM signaling
Checklist
1) Phone is GSM
2) Phone is unlocked if you want to use foreign SIM card
3) Phone is quad band
I use T-mobile: in FR and Canada texting between myself and herself are included at no charge. $0.20 per minute for roaming phone calls. UMA IMA VOIP calls at no charge. 3G Data also included while roaming