I'm interested in getting a smartphone but I'm not sure which to buy. I'd like to stay on a GSM network since I travel internationally, so that means staying on AT&T or T-Mobile. I don't care about bells and whistles so long as I can check my email in a reasonable interface. If I get a wifi enable phone, can I opt out of my service provider's network and just use available wifi for internet and email, or do I have to subscribe to whatever email network service?