Operators True Move and Digital Phone (DPC) will have to migrate customers off their 1800-MHz GSM networks and onto other networks so that the spectrum can be re-issued for 4G services, the Bangkok Post reported.
If they fail to do so, the signals for 2G SIM cards on mobile networks operated by the two companies will be cut off after September 15.
"The two operators have to transfer all existing 2G customers to other networks by July before the 4G auction takes place," said Takorn Tantasit, secretary general of the NBTC.
The 2G licences expired in September last year but they were granted a 12-month extension.
The NBTC expects to hold an auction for two 4G licences using that same spectrum in August.
One 17.5MHz licence in the 900MHz band is scheduled to be auctioned in November, while a third auction for 50MHz of 1800MHz spectrum will take place in September 2015.
