I've seen this before.
Months ago exactly the same system was set up over here.
But nothing is sent to any mobile phone, nor is there any need to store any private data: The only thing one needs to measure is the frequnecy of handovers.
A handover is when a mobile phone in a cellular systems hops from one cell to the next. So when you measure per cell the amount of phones comong into the cell and the amount of phones leaving the cell, you get a very good idea of the traffic:
If incoming is about equal to outgoing then traffic is flowing.
If incoming is larger then outgoing then there is a buildup of traffic, the start of a jam
If outgoing is larger then incoming, a jam is dissapating
If the freqency of handovers is high, the traffic rolling going fast
If the frequncy of handovers is low, the traffic is rolling slow
(So it is like measuring pressure and flowspeed of a fluid)
Again, Note that there is no real reason at all to collect individual IDs.
The data gained this way is simply combined with data coming from other measuring techniques (loops in the road most of the time) to control electronic road signs and the classic radio traffic announcements and TMC.
The set up over here was specifically aimed at tunnels: there it is very easy to measure traffic since the inside of the tunnel is usually one cell and both ends are two other cells.