Someone told me that if you register you phone number at one of the local phone shops in Tesco that you can then use it to call back to Thailand while in Malaysia.

Does anyone know which shop can accomplish this.

Also, if not, what other options are there for calling back to Koh Samui from Maylasia.

You have to set the phone up for international roaming. I did it through the call center.

I did this with Telewiz in Tescos on a 1 -2 Call AIS Service

Malaysian sim cards are very cheap & it is only 1 baht to call Thailand. Found it a lot easier than using AIS

A sim card in Malaysia 200thb includes 100thb credit.

If you use a Thai sim in Malaysia you will be also charged if people call you.

You also need to take some ID to register the sim card at the shop.

The easiest way sounds to just buy a Malaysian sim card. Especially cause it's only 1 baht a min. to call back to Thailand.