Calls within Thailand can run from 1 to 5 baht per minute. The variation is due to various customer selectable calling plans; to one number, on the same network, time of day, shorter expiration date, etc.
One-2-Call subscribers can dial *777, then press 1 for English, to hear descriptions of all the current calling plan promotions. During the call you can identify your current plan and change to a different one.
Calls to the US/UK/OZ run 6 to 9 baht per minute. The 009 prefix (009 44 nnnnnnnnnn to say the UK) is 6 baht per minute, standard rates (+44 nnnnnnnnnn) are 9 baht per minute. These speecial rates were extended until the end of this year, not sure what the status is after that