I found this to be a very interesting article on ISIS with insights to their communications,

http://www.theguardian.com/world/2014/dec/11/-sp-isis-the-in...

When people left Bucca, they had written each other's names & numbers on the elastic of their underwear.

They also responded to Anonymous' threat toward ISIS by suggesting techniques to secure Comms and what to avoid.

http://www.businessinsider.com.au/isis-anonymous-statement-2...

I would suggest that this cell did not use protection but I am confident the larger ISIS network is not so transparent. My suspicions would be that most Comms is via word of mouth.