People holding more than 100 mobile SIM cards must register them all under their name by Wednesday, or risk seeing the numbers terminated.
The National Broadcasting and Telecommunications Commission (NBTC) has warned that owners of 100-plus cards have until Feb 14 to verify their registration, and holders of from six to 100 numbers must do so by July 13.
The telecom regulation agency hopes this will reduce the number of call scams plaguing the country.
People failing to meet the deadline will not be able to make outgoing calls on those numbers. They will have another 30 days to comply, and then the cards will be permanently cancelled.
Mobile phone users can verify the numbers at all customer service desks, or on mobile operator apps.
"The NBTC realises this measure can cause inconvenience for some customers. However, verification can reduce the chances of criminals using the SIM cards for wrongful purposes or connecting them with accounts opened for use in scamming," NBTC chairman Sarana Boonbaichaiyapruck said on Tuesday.
There are about 7 million numbers owned by holders of more than 100 cards and only 1.6 million are registered, as of Feb 11, according to the NBTC. There are almost 4 million numbers held by people with more than five cards, and only about 300,000 of them have been registered, the telecom regulator said.