I've got a simcard from Cambodia that won't work here in Thailand. If I try to connect manually with the 2 operators on my phone (True 3G+ and AIS) I get the message "NO NETWORK ACCESS". I called with customer service of Cellcard in Cambodia and they told me that the problem could be caused because my handphone cannnot connect to the 3G network.

Could this be true and should I cut my simcard and put it in a smartphone?

3 from the UK doesn't work in Thailand either (works fine in Laos).

I think True (which seem to have the roaming contracts) have a problem they won't admit.

PS, I called 3 in the Uk, same runaround.

Buy a new SIM for Thailand.

Seems like it should work provided you've registered, have a compatible device, have coverage and have balance? Looks like they have agreements with AIS, DTAC and TrueMove H. Not sure which bands are supported, they aren't detailed, but suspect, at a minimum, you'd need 3G/850 MHz (Band 5) or maybe BAnd 1 (2100 MHz).

It will be expensive. Unless you need to receive in-coming calls it might be simpler to get a local Thai SIM?

Can you share any details?

http://cellcard.com.kh/outbound-roaming

Not sure what kind of device you have, knowing these little things always helps when trouble-shooting, but you should normally try to "connect automatically".

Seems like it should work provided you've registered, have a compatible device, have coverage and have balance? Looks like they have agreements with AIS, DTAC and TrueMove H. Not sure which bands are supported, they aren't detailed, but suspect, at a minimum, you'd need 3G/850 MHz (Band 5) or maybe BAnd 1 (2100 MHz).

It will be expensive. Unless you need to receive in-coming calls it might be simpler to get a local Thai SIM?

Can you share any details?

http://cellcard.com.kh/outbound-roaming

Not sure what kind of device you have, knowing these little things always helps when trouble-shooting, but you should normally try to "connect automatically".

I had put the simcard in an older mobile phone. Yesterday I went to a TRUE shop, they cut the simcard and put it in my smartphone and I got connected. Problem solved!

I suspect that your "older mobile phone" only supported GSM 900, which is no longer supported here in Thailand. Your smartphone obviously supports one (or more) of the local 3G bands.