T-Mobile charges standard prepaid rates for Wi-Fi UMA calls. With their $1/day plan (no monthly fee), international calls back to the US cost $0.10/min. Even though I'm an AT&T customer, I bought a T-Mo phone with Wi-Fi just for international use (turning off the GSM network). Unused prepaid funds expire in 90 days, or 1 year if you buy $100. Good for US conference calls.