AIS, Thailand's largest mobile operator, announced that it was ready to expand its service to integrate 10-digit dialing.
Thailand's National Telecommunications Commission (NTC) earlier ordered mobile phone companies to upgrade their customers' SIM cards to prepare for the new system within the next three months.
The new system will increase the potential number of telephone numbers from 90 million to 300 million, to meet growth over the next 30 years.
AIS subscribers, numbering about 17 million, can upgrade or replace their SIM cards at any of 600 of the company's branches across the country, starting next month until November, AIS said.
AIS is also providing mobile caravans to help customers in remote areas with the changeover.
If users are not able to switch to the new system by the deadline, they will not receive incoming calls.
AIS customers who use Symbian phones can edit their mobile numbers online by visiting the company's Web site and downloading the program.
New subscribers are automatically being put on the 10-digit system from next month with a new type of SIM card.