Interior Minister Nisar Ali Khan last week set the deadline for the top five telecom operators - Mobilink, Ufone, Telenor, Warid and Zong. He said the government did not want "free availability" of SIMs to militants.
The Taliban militants who attacked the army-run school in Peshawar on December 16 that killed 150 people, most of them children, used a SIM (subscriber identification module) of one of these top five telecom operators.
He said the operators needed at least 150-200 days to complete the re-verification process.
"We want a realistic period for the verification process," he said, adding the general public would be the ultimate sufferer if the SIMs were blocked for a month.
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"The affected persons will rush to customer sale centres, franchises and retailers to get post-paid SIMs," he added.
According to the Pakistan Telecommunication Authority, there are a total of 140 million mobile phone users. Of them 125 million are pre-paid and 15 million are post-paid.
Telenor Pakistan official Mohammad Aslam Hayat said: "There is no such thing as unverified SIM. Every mobile operator has a record of SIM sales from its warehouse up to the retail level, where subscriptions are acquired.
"Against every SIM sold, mobile operators have necessary data as per the regulatory requirement. However, this does not mean that the system is foolproof."
Hayat said the "dominant buyer" of bulk SIMs was a group of people involved in "grey" international incoming calls.
The PTA has called a meeting of all operators tomorrow to discuss a solution to the problem.
"The Pakistan Rangers has also written to the PTA that the regional head of a cellular company of the SIM found in the use of a terrorist will be responsible," an official said, adding the warning has created panic among the operators.