Thaitel is offering free calling to anyone in Thailand if you subscribe. They have Happy Hours everyday during which all calls to Thailand are free. These hours are for 2 hours daily and change according to calling card plan you choose to purchase. The best calling cards are Be Smart ( Happy Hours 1:00 PM – 4:00 PM US Pacific Time, Call Anywhere ( 11:00 AM – 2:00 PM US Pacific Time ) which have free calling to Thailand early hours. More info about other Calling Card plans here.
Thaitel offers all their services via SIOP (Service over Internet Protocol). So you can use your Nokia SIP Phones or VOIP Adapters to configure. They also have their own branded SIP Softphone. Thailtel has very reduced rate to 2.5c/min to call thailand, Bangkok from USA. Download the software here.