Getting international telephone calls costs much in Thailand. At the arrival terminal of Swarnabhumi airport, you can buy a mobile phone chip with connection for 200 Baht (with free calls of 50Baht) fron phone company stalls and insert in your phone to dial international numbers. But when you enter Pantip Plaza or even travel alone some market places you will see street vendors sell same thing for 25-30 Bahts. One international call will cost you about 24 Baht. =  0.60 US$ while local call is 3 Baht. The pre-paid connection will automatically cut after 6 days unless you recharge it for further period. 

Also, there are many phone cards available and phone booths are in many places in towns. You have to dial a connection number to get international call ( 008, 007,,,,) before dialing your country number. They also charge about 24 Baht for an international call.

Best thing is go to an internet cafe and use net-to-phone or other internet  facility to dial to your country.

Please note:  That if you have an old local mobile phone number from before that you want to call to or have gotten a mobile telephone number from a printed or old source. You will need to add the number "8" after the first zero. So the second digit of all handphone numberswill be an "8" from now on. There are just so many handphones in Thailand it had to be done.