I always use Skype for international landline or mobile calls as their rates are extremely low (as little as a few cents per CALL). I loaded $10 about 3 years ago and still have a $4 balance!
Use an agent in Thailand, there are tons of them so shop around, and ask for a retirement visa and some of them can also assist you with the 800k baht requirement. Cost is around 30k baht per visa and is valid for 15 months & renewable yearly.