The French intelligence and counter terrorism units are either not doing their best or these terrorist are getting much better at covering their tracks.
If it is a small group of just half a dozen people you should be pretty much invisible if you act a bit clever. Coordination can be done via throw away sim cards and personal meetings so you don't really need to communicate that much.
You forget the fact that the communication channel is always being monitored, but in this sad case, nothing was caught. Someone has already smuggled the weapons in before hand, just like how the bomb was able to made it to the plane early this month. Someone must have been tipped and bought the weapons from possibly local gangs and bam...
'Chatter' is a sign of stuff not working as expected, if terrorists are half as intelligent as I give them credit for they'll know to stay off the phone and off the internet. You'd have a bigger chance locating them by the absence of traffic than by traffic assuming they are not as dumb as we'd like them to be.
Incidentally, and violating my own rule, that's why I don't believe in the whole 'snooping makes us safer' rubbish. If anything it just increases the size of the haystack for a constant number of needles.
Particularly re. phones / internet, it only takes one mistake to link a phone number used for nefarious activities with your identity.
(I would expect them to use phones, at least - they have to coordinate somehow.)