Sorry if this reads like an ad, but I thought I'd publicise http://www.18185.co.uk
They charge only 1.5p per minute to Thailand, to a mobe or a landline, and offer access from a BT phone, a cable phone, Talk Talk, your mobile, etc etc.
I use it every time i talk to my waan jai.
Read their website and see what you think. They have never ripped me off (which is more than i can say for my gas and 'leccy companies)