Pakistan recently announced that there were four million unidentified (many presumed illegal) SIM (Subscriber Identity Module) cards in use for cell phones nationwide. Since the owner of these SIM cards is not known, such unidentified SIM cards can be used by outlaws avoiding detection. The government could order all these SIMs disabled but that would cause an uproar because most of them are being used by ordinary citizens rather than gangsters or terrorists.
In response to this situation, the government ordered SIM card vendors to only deliver new SIM cards via mail rather than by hand. This is probably unworkable because people often need a new SIM card right away because the existing one is now useless and they want their cell phone operational right away. People are supposed to carry ID with them, but fake ID is easily available, at least to criminals and terrorists.
Since 2008, Pakistan has tried, without success, to block the anonymous use of cell phones. This usually involves unidentified SIM cards but now there are phones modified to operate without the mandatory IMEI (International Mobile Station Equipment Identity) numbers. Such phones tell the phone companies nothing about the phone.
In 2012 the government had to back off on threats to block calls made by someone who bought a SIM for their cell phone using unidentified SIM cards. The government has not been able to come up with a foolproof system that would prevent criminals and terrorists from buying SIM cards that could not be traced back to the purchaser.
Back in 2010 Pakistan shut down cell phones owned by anonymous users. This turned out to be a huge number, 11 percent of 88 million SIM cards in use at the time were unidentified. The shutdown threat caused such a public uproar that the government had to back off. In 2009 there was a problem in the tribal territories with dealers who were not obeying new rules that required them to get positive ID from cell phones buyers. In that year the government blocked the use of 10.5 million unregistered SIM cards for cell phones. That caused another public uproar and the bad guys figured out how to get around that.
These SIM cards are the type long favored by gangsters and terrorists. In most parts of the world you can move your cell phone service from one phone to another by simply removing the small (25x14mm) SIM "card" from one phone and inserting it in another. SIM cards can also be bought just for the minutes stored on them. Police have long noted that terrorist bombs are often set off using a cell phone with an anonymous SIM card. It's not uncommon to raid a terrorist hideout and find hundreds of anonymous SIM cards.
By 2009, there was already a thriving Pakistani black market in SIM cards that were registered with phony information. The Taliban and drug gangs knew of their vulnerability and could not run their operations without these communications tools. Criminals in other parts of the world have developed ways to lessen SIM card vulnerability and the terrorists are learning as fast as they can. In the meantime it's getting them killed or captured when they use easily identifiable SIM cards.
It was believed that, with anonymous cards more difficult to get, it would be easier to track down bombs that use cell phones for detonation. But Pakistan is a very corrupt place and merchants were able to circumvent the new regulations by simply bribing any police who came by to check SIM card registration. The cell phone companies also resist these government crackdowns, as it is bad for business.