As Alan said, why deal with getting an METV. Why not just come in on a 30 day stamp, if you want to stay longer, you can extend it at immigration for 30 days for 1900 baht. If you want to leave Thailand and come back, no problem. You get a fresh 30 day stamp when you re-enter and start process all over. For 3 months, this seems your best option.
Sprint and Verizon, At&T don't work here. Different frequency, or something like that. I keep my Tmobile phone active to receive texts needed for verifications to log in to online accounts. Also, as back up data service when I need it.