The national telecom regulator plans to insert a "1" in the middle of the existing two-digit prefix of fixed-line numbers in order to increase the number of fixed-line and mobile numbers by 550 million.
Numbers starting with 02 will become 012. Calls made to fixed-line numbers in provinces will have to add a 1 as the second number of the existing three-digit prefix, so that 053-123 456 will become 015-312 3456.
"The planned three-digit prefix of fixed-line numbers is scheduled to take effect in 2021," said Prawit Leesathapornwongsa, a commissioner of the National Broadcasting and Telecommunications Commission (NBTC).
Up to 500 million numbers would be reserved for the mobile market and 50 million numbers for fixed-line service.
The NBTC's telecom committee approved in principle the new three-digit prefix for fixed-line numbers on Tuesday, said Mr Prawit.
He said the NBTC will set up a special working panel to draw up a final draft for the new fixed-line numbers, before submitting it to the telecom committee again. A public hearing will be held after the committee hearing.
The final draft is expected to finished by early 2017.
Mr Prawit said TOT Plc, as the concession owner of fixed-line telephone service, will inevitably be affected by the new numbering system. The state telecom enterprise would require investment of 6.5 billion baht to upgrade its transmission system in order to service the NBTC's plan, he said.
The new numbering platform aims to serve the explosion of wireless and fixed-base broadband markets, where demand has been driven largely by the emergence of Internet of Things applications, said Mr Prawit.
As of April, the NBTC had allocated 179 million numbers to mobile operators. The regulator has another 88 million numbers available, which should be sufficient until 2020.
Demand for new telephone numbers is projected to increase significantly, driven mainly by the country's digital economy development and the national e-payment scheme, he said.