Hi,
Vyke https://www.vyke.com/ gives you calls to Thailand for less than 2p /min, just over 1p for sms. Costs depend on how you use it, there are many options!

Fring http://www.fring.com/ allows you to use you Nokia mobile with Skype, Google talk and msn using data. If you have a web and walk package (T mobile) it could be very cheap.

Watch your data tarrif as most are a rip off

Free sms to Thailand, Canada and a few others with Yahoo messenger, but don't register yourself as living in the UK. I use the Singapore version of messenger. I live in the UK and it works.