Just read a little bit about a 009 prefix for making calls to UK.

7 baht a minute?

How does this work and can I use on GSM?

Yes you can use it on your mobile, and it's worked for everone I know who's tried it.

Just dial 009 44 xxxxxx rather than hitting the + key.

If 009 doesn't work try 008 then 7, 6 etc. all are cheaper than the default 001.

I believe that 009 uses VOIP to make your call (that's why it's cheap), this does mean that text messages do NOT work.

I found this somewhere, and saved it...

To use any of the services, simply enter the three digit number and then the country code you wish to dial. Your charge will be added to either your CAT bill, your TOT bill, or will be taken from your cellphones pre-paid minutes.

001 ... is the most expensive direct-dial service. It works from al phones and should be the highest quality connection. We do not recommend ever using this service as it is unnecessarily expensive.

002 ... is a TOT service designed to call only to Malaysia. It has two rates; calling between 7am and 6pm costs 11 baht per minute and between 6pm and 7am costs only 7 baht. This service does not work from cellphones.

007 ... is a hugh quality connection which can be used on TOT and TT&T lines only. Call to the following countries for 9.63 baht per minute (including 7% tax). It works to the following countries:

China, Germany, Hawaii, Hong Kong, Japan, Laos, Malaysia, Singapore, Taiwan, UK

008 ... is a TOT-only service which offers a flat rate of 7.49 baht per minute (including 7% tax) to 30 destinations:

Alaska, Australia, Belgium, Brazil, Brunei, Canada, Chile, China, Columbia, Denmark, Ecuador, Finland, France, French Guiana, Germany, Greece, Guam, Guyana, Hawaii, Hong Kong, Indonesia, Israel, Italy, Japan, Kuwait, Paraguay, Peru, South Korea, Macau, Malaysia, Mexico, Norway, Russia, Singapore, Suriname, Taiwan, UK, Uraguay, USA, Venezuela.

009 ... This is a CAT service which can be used from every phone. This is also known as the efone service. Rates for popular countries are now only 6.49 baht. Check our cost calculator for more details or click here.

I would modify what crossy said and tell you to enter it as +009 then your country code and then number. This is how it's worked for me when calling on both the DTAC and Orange networks. I have yet to get through on the 008 or 007 routes, in spite of my DTAC brochure listing the numbers as available and cheaper than 009.

Thanks very much for that.