Just went to my local post office and picked up a form about the post offices phonecards when i got to the cashier she saw i had the form and said oh we have some free trail international phonecards you can have, she gave me 4 saying they probably will only last a few minutes each.
I gave it a try phoning a mobile in Patong and the card said i had 80 minutes left of free calls
Worth a try to ask see if you can get one from your po.
I am no expert on phonecards but the cheapest international card they do is £5 but calls to Thailand range from about 3 pence to 7 pence a minute which does seam cheap to me.