All fixed-line telephone numbers will be 10 digits nationwide by 2021.
The present nine-digit numbers will get a number 1 after the prefix number 0 of the provincial code, starting in the third quarter this year and concluding by 2021.
By adding one digit, the supply of phone numbers will rise to 550 million, of which 500 million are for mobile service and 50 million for fixed-line numbers.
The move is aimed at dealing with continuous mobile service demand growth.
The National Broadcasting and Telecommunications Commission (NBTC) has assigned 260 million numbers to mobile phone operators. Some 52 million numbers are left, estimated to last until 2020. The NBTC has also assigned 50 million numbers for fixed-line service.
A meeting of the digital economy and society (DE) minister, management and NBTC commissioners last weekend in Hua Hin agreed to work closely on regulatory framework and digital infrastructure to enhance the country's development and digital economy.
Pichet Durongkaveroj, the DE minister, said the NBTC and DE Ministry need to work on fixed line transformation quickly because phone number demand will increase in line with digitally-driven development.
Part of the demand will be driven by an expansion of wireless and wired broadband services for machine-to-machine connectivity of smart devices, the Internet of Things.
Mobile numbers cannot have prefixes of 02, 03, 04, 05 and 07 because of limited transmission settings, he said. Nine-digit numbers can still be reached during the transition.