This has nothing to do with phone security though. Pakistan is the source of spam calls in many developed nations. Those spam call center operators was able to operate on the cheap from Pakistan due to cheap labors and cheap access to international calls via VoIP, so by blocking unregistered VoIP and VPN, they hoped to kill the spam call center industries (or at least that's what they tell people when they started cracking on vpn a few years ago, might be legitimate if they're getting pressure from western goverments to control the spam situation). This will also increase tax revenue because legitimate call centers will have to use licensed VoIP services that pay tax to Pakistan government.