I'm sure this must have been answered before.

Can you keep an existing phone number when changing your mobile phone network supplier on a prepaid basis?

I am currently with TrueMove but have an HTC phone which won't allow 3G access on the TrueMove 850MHz band. The True signal is also unreliable at my home.

I would prefer to keep the number I have had for many years, especially as it's a good easy to remember number, but it's not a major disaster if I have to change it.

http://www.ais.co.th/faq/en/MNP.html

Insure that you are the registered owner of the SIM/number with Truemove. If you haven;t done this yet go to a Truemove shop/office to register (name/passport), then wait 7 - 10 days for the information to get to the clearinghouse. Then go to an AIS shop to initiate the MNP process. You will lose any remaining balance with Truemove and have to pay 99 baht for a new AIS/One-2-Call SIM.

This Truemove MNP page, while not directly applicable, has a description of the process in English.

http://www.truemove.com/mnp/en/know_mnp.html

Thanks. Very helpful.

Great thanks, looking for this for a while

The number must also be active on the provider you are transferring from, not suspended. AIS one day suddenly decided I wasn't spending enough and suspended my number. True took the fee to transfer the number to one of their SIMs, but didn't notify me when the transfer failed, I had to enquire and get them to translate the failure code (took about an hour to get it out of them). I let the number go rather than pay AIS to reactivate the number and then try transferring again.

So, make sure you start the transfer well before the number expires and enters that limbo where it isn't yet resold, but can't be transferred.