A few months ago my Belgian SIM card stopped roaming, I tried it in 3 other phones including one at a shop in Tukcom but it was dead in all 3.

OK, so it's the SIM card not the phone. I contacted my SIM card supplier in Belgium and they said there was no problem with the SIM card and that their roaming partners in Thailand were AIS, True and DTAC.

Twice I went on trips to Indonesia and roaming worked perfectly until I returned to Thailand. AIS in Central Festival said their roaming was working fine.

Anyone else had roaming problems here? Have the government blocked roaming?

I did not notice some problem of roaming . Sometimes difficult to get the thaï networks but works perfectly when have

Data, Voice or both?

While roaming agreements can and do change, you've checked and stated that isn't the case.

So, I might suggest you look at what bands/frequencies your PHONE supports, and what bands/frequencies your roaming partners currently operate on.

Thailand has recently shut down and is auctioning and 'refarming' TWO major frequency ranges. Both the AIS-concession 900Mhz (band 3), as well as the AIS and True Move concessions on 1800 (band 8) were turned off this year as their licensed operating frequencies were put up for auction. Most likely your phone only worked with one of these operators on the frequency/band that was recently shut down in preparation of the auction.

As of November 2015:

850 (band 5) ...DTAC, True Move H and MyCAT operate 3G services here (limited 2G GSM availability remains)

900 (band 3) ...recently vacated (or in the process) by existing operator AIS (2G and 3G service), waiting for mid-December 2015 auction outcome

1800 (band 8) ...DTAC currently operating 2G/4G/LTE (note no 3G)

AIS and TRUE Move H were recently awarded operating licenses in this spectrum and are currently planning deployment of towers and infrastructure for 4G/LTE operation

2100 (band 1) ... AIS/One2Call TOT-3G both provide 3G service, DTAC/Happy and True Move H currently operate 3G/4G/LTE services here

Note: 3G/4G/LTE normally only relate to the 'data' side of mobile provider services, Voice Call service may require 3G/4G/LTE compliant 'radios' if 2G/GSM-only services have been terminated on the connected tower.

Thanks for the info RichCor. Seems Thailand is changing their phone systems.

However, having tried my Belgian (Proximus) SIM card in three phones I doubt that the phone is the problem. Also, as Proximus has 3 partners in Thailand I would expect one of them to be working on roaming at this time.

My last chance is a new SIM card which should arrive in a week or two.