The telecom regulator is considering the use of 11-digit mobile numbers instead of the current 10-digit ones in anticipation of a number shortage.
The switch to 11 digits would require users to add an extra digit to the existing three-digit prefix, followed by the existing seven-digit number.
The new format should increase the amount of telephone numbers by several hundred million.
Takorn Tantasit, secretary-general of the National Broadcasting and Telecommunications Commission (NBTC), said the plan to move to an 11-digit system is a response to complaints by operators that they are running short of numbers, spurred by 3G migration and the spread of internet-connected devices.
"Thailand is likely to face a mobile number shortage if the number of mobile users continues to grow exponentially," said Mr Takorn.
The NBTC board on Wednesday approved a draft numbering plan for telecom service, a revised version of the current regulations. A public hearing on the draft comes next month, and the rules would go into effect in early 2014.
The draft numbering plan will govern numbering and numbering management plans for mobile services, fixed-line phones, SMS texting and international dialling.
The draft is to be revised every five years to serve the market and encourage free and fair competition in the industry.
Mr Takorn said 140 million mobile numbers are available now. Of the total, 100 million are reserved for the 01 prefix and the rest for the 09 prefix.
"If the 01 prefix is used, the industry will face a mobile crunch situation," he said.
The NBTC is urging mobile operators to use numbers properly and rotate the existing numbers more effectively.
Mobile operators pay two baht per number as a monthly numbering fee to the regulator.
Mr Takorn said the NBTC plans to ease restrictions on the returning of unused numbers by setting a minimum required rate of return of 1,000 numbers instead of the current 10,000. The new condition is to tackle the many unused mobile numbers in the system.
He acknowledged that the surplus of unused numbers arose because operators must keep inactive numbers to comply with rules barring expiry dates for prepaid mobile service.
Mr Takorn said switching to the 11-digit system could cost the NBTC some 100 million baht to build public awareness.