NBTC secretary general Thakorn Tanthasith said however that the extension of registration until August 31 would mean only those who failed to register their cards by the July 31 deadline could answer calls but could not make calls during the extended period.
But he said these callers could still call 191 emergency numbers only.
After the expiration of the extended period on August 31, they could neither receive or make calls, even the 191 emergency numbers, he said.
The extension of registration of prepaid SIM cards came after 16.9 million users of the prepaid cards still did not register.
He said there are altogether 85.5 million prepaid cards in use but only 68.8 million have registered by the July 31 deadline, or 80.2%.
He said the remaining 16.9 million users have until end if this month to register otherwise they could not be used both to answer or make calls.
He said it was necessary to require registration of all prepaid cards for security reasons as some prepaid cards were used for criminal activities such as drug trafficking and insurgency.
Of the total 68.8 million registered cards, 34.3 million are users of AIS mobile phone service, DTAC 21.2 million users, and True 12.9 million users.