Over the past years there have been massive changes to all the major operators’ networks. Before you purchase any SIM card, please make sure that the network you are choosing is compatible with your devices (see below). The big three providers have separate networks operating on more than one set of frequencies that are combined through domestic roaming. So you should better enable roaming on your device, even for domestic use. Most old networks mainly on 2G are about to decommissioned.
AIS has 3G/DC-HSPA on 900 and 2100 MHz and started in 2016 with 4G/LTE Advanced called “4.5G” on 1800 and 2100 MHz
dtac has the largest 3G spectrum: 3G/DC-HSPA on 850 and 2100 MHz and 4G/LTE on 1800 and 2100 MHz.
has 3G/DC-HSPA on 850 and 2100 MHz 3G/DC-HSPA and the largest 4G/LTE spectrum on 850, 900, 1800 and 2100 MHz
CAT: is on 3G/HSPA only on 850 MHz
TOT: is on 3G/HSPA on 2100 MHz and plans 4G/LTE on 1800 MHz 4G/LTE and 2300 MHz TDD-LTE.
For 4G/LTE in 2016: True has the best coverage at 90% of population, followed by dtac that now covers all 77 provinces today but lacks coverage, while AIS has only started to roll out 4G, partly as “4.5G” with carrier aggregation, also covers 77 provinces but also lacks coverage too.
Note that all old 2G/GSM networks are about to be shut down in the next years as the providers need more spectrum for LTE. AIS has sealed a roaming agreement with dtac for 2G on their 1800 MHz band, which dtac is going to use until 2018. True has leased back bandwidth from CAT on 1800 MHz for 2G until the end of 2016 while TOT has never had any 2G. So be prepared that 2G is coming to an end soon in Thailand and better bring a 3G or 4G/LTE device.