Several ways.
A regular phone (with a contract) requires address information and (usually) a credit check. Not something your average terrorist wants to leave lying around - even in itty-bitty pieces.
Two: as the terrorists know (thanks to the MSM), we are monitoring cell phone calls. If I were a terrorist and needed to use cell phones, I'd have a large stock of them and switch frequently between them. Again, not something you can easily do with regular phones.
Cloning any cellphone is simple, and has been going on since forever.
I guess i just know & understand more than most of you to not see this as much different than them obtaining regular cellphones through other methods, like a bogus 'phone donation to a womens shelster" dropoff point.
I'd bet over 100 cellphones are "lost" or stolen every day. I would much rather use a stolen phone than a bought pre-paid phone for terrorist purposes, we i a terrorista.