Might sound unbelievable, but you should take into consideration that most of these terrorists will use whatever means to avoid being tracked. After raiding the terrorist coven, the police found a PS4 console that many believed was used by the terrorists to communicate.
Forbes suspects that one major reason the Paris terrorists succeeded was because they used Playstation 4 consoles to communicate. The Belgian federal home affairs minister, Jan Jambon also confirmed this, according to him:
The PS4 is used by ISIS agents to communicate, and was selected due to the fact that it’s notoriously hard to monitor. “PlayStation 4 is even more difficult to keep track of than WhatsApp.”
The PS4 might be the best bet given that there is an IP-based voice communication feature which is much more difficult to monitor.
For example, on the PS4, you can voice chat, message each other, or even write temporary messages on walls in a game like Call of Duty.
One thing is certain, if this is what they used, no level of monitoring can stop them from communicating with each other.