Traffic analysis
It's nice if you know the content of the messages, but knowing who sends what when is a good start. The British used it in WW2 already for Axis radio messages, with unusual message levels or message lengths being indicators for higher priority for deciphering.
The Da'esh know this, and therefore use a phone just a few times before dumping it, so well before any of the spook organisations would become aware of it being used for nefarious purposes. The fault there was not using plain text messaging, the fault was dumping it where it could be recovered and too close to where the attack took place. Recovering the phone led to checking when it was used and where the messages were sent from/to, which again led to the raid in Saint Denis.
Biting the hand that feeds IT
