Since a friend of mine recently visited Thailand I participated in a discussion on
this forum concerning 3G frequencies and compatible devices. Afaik the following are the standards and frequencies currently used in Thailand, which actually includes one operator (TOT3G) using UMTS2100:
•AIS:
GSM 900, GSM 1800, UMTS 900
•TOT3G:
GSM1900, UMTS 2100
•DTAC:
GSM 1800, UMTS 850
•TrueMove:
GSM 1800, UMTS 850
According to
this article an auction for 45 MHz of 2100 MHz spectrum will be held in September or October 2012, which according to
this article shall be auctioned in blocks of 10 and 15 MHz and so would allow 3-4 operators (probably all existing ones) to acquire 2100 MHz spectrum.