Where did this researcher get the anonymized mobile phone location data points from ?
Also, I think it's important to clarify that:
"Every cell phone user has to be tracked so the cell network knows how to route their calls ..."
... would be more accurate if we said:
"Every cell phone has to be tracked ..."
Many pieces of the cellular network have no idea who the handset (or other device) is assigned to, or who is using it - they just know that that SIM card is up on the network, and needs to be routed appropriately ...
Of course the telecom itself, for many purposes - not the least of which is billing - needs to know a personal identity behind a SIM card.
What I am saying is that individual cell sites - and other pieces of the cellular infrastructure - are only indirectly (or not at all) connected to personal identity of the mobile subscriber. Call routing can be done just fine without any idea of subscriber identity in meatspace.
Also, I think it's important to clarify that:
"Every cell phone user has to be tracked so the cell network knows how to route their calls ..."
... would be more accurate if we said:
"Every cell phone has to be tracked ..."
Many pieces of the cellular network have no idea who the handset (or other device) is assigned to, or who is using it - they just know that that SIM card is up on the network, and needs to be routed appropriately ...