Constantly changing which cellphone you use definitely helps if you're a terrorist trying to avoid detection, but it turns out that it's not gonna get you very far if you keep using the same SIM card in each phone, which is apparently how the US was able to capture Khalid Sheikh Mohammed, Al Qaeda's military commander. Even buying anonymous, pre-paid SIM cards didn't make a difference, and in fact was what lead to their downfall. Because they could be purchased anonymously, the terrorists didn't think it was important to switch cards from time to time.