Consider T-Mobile and include Hotspot at Home.

T-Mobile will unlock the phone for you so you can use a local SIM card.
And/or... if you frequent places with wi-fi, the phone will operate using local Wi-fi (any service you don't need to log in to - or T-Mobile's service). While on wi-fi, you get unlimited usage & any calls initiated while on wi-fi remain free even if you roam to GSM (although those started while on GSM remain billable even if you roam to wi-fi). I don't know if wi-fi remains free if you use a non t-mobile sim.