After living over 3 years in bangkok and spending way too much money on international phone calls, i recently found a good website on how to slash those phonebills: www.sawasdeetalk.com.
This thainglish website offers all kind of stuff for cheap international phonecalls to and from thailand and the USA.
They have Thai phone cards (they call it a white card), a free softphone and the latest VOIP technology: a free SIP phone.
Recently they even added free calls to 34 countries.
I would say; check it out.
I already cut my phone bills in half, using that SIPphone.