Signals for 2G SIM cards on the 900-megahertz network will be cut off after midnight Tuesday, says the National Broadcasting and Telecommunications Commission (NBTC).
About 400,000 2G users remain on the 900-megahertz network run previously by Advanced Info Service (AIS), the country's biggest mobile operator.
The cut-off order came after the NBTC Monday granted a 15-year 900MHz spectrum licence to True Move H Universal Communication (TUC), the 4G arm of True Corporation, three days after the company paid for the licence.
This was in line with auction rules stipulating that AIS would have to switch off its network three days after bid winner TUC paid for its licence.
Also Monday, AIS filed a complaint with the Central Administrative Court, asking the court to force the NBTC to extend the period for its 2G customer retention to ensure service continuity and consumer benefits as stipulated in NBTC rules.
The court asked the NBTC to provide more details about customer retention measures Tuesday.
"AIS will have one last chance Tuesday to migrate its existing 2G users to other networks or to brace for customer losses," said NBTC secretary-general Takorn Tantasith.
He said 400,000 2G users on the 900MHz network will be unable to use mobile services after signals are cut off.
AIS has already migrated 7.6 million 2G customers to DTAC's 1800MHz network under a roaming agreement.
Mr Takorn said AIS insisted that DTAC's 1800MHz network, with 98% population coverage nationwide, could accommodate the company's 7.6 million users.
He also stressed that the NBTC has no authority to extend the period for AIS to oversee its 2G customers, saying extending it would be breaching the telecom laws.
"We did our best effort to seek a win-win resolution among the three related parties on the 2G customer retention deal, but no one could accept the proposals of each other," said Mr Takorn.
The three parties involved are AIS, TUC and TOT Plc.
AIS chief executive Somchai Lertsutiwong said the company was preparing to give away 3G/4G smartphones to its 400,000 2G customers faced with losing phone signals.
Customers who bring their disconnected 2G SIM cards will get a free smartphone handset plus 50 baht worth of call time. The deal will be available at AIS service outlets and the Subdistrict Administrative Organisation nationwide.
AIS has spent at least 8 billion baht giving away 4G smartphones to its 10 million 3G customers still using 2G phones including the latest campaign.
The budget also includes the cost of AIS renting DTAC's 1800MHz network to continue providing 2G mobile services.
Mr Somchai said AIS wanted the NBTC to allow the company to automatically migrate its 400,000 2G users to AIS's 3G network.
"If the regulator accepts our request, those 2G users will face no service disruption," he said.
Mr Takorn, however, said the NBTC rejected AIS's request as it would be breaching the regulations governing mobile number portability.
Mr Somchai also said TUC's proposal to give AIS another three months to oversee the latter's 2G users on the 900MHz network free of charge was simply to build its corporate image because accepting it would be breaching telecom laws.
More importantly, he said, the proposal could not be concluded before the network shut down.