If I had a choice between a full recording of all their phone calls, or a trace of their movement with 50 meter precision? Good chance I'd go for the second one to be honest - unless I knew for sure they made a lot of voice calls.
Metadata from one person isn't worth anything, but the graph of who's contacting who is very useful. Plus, it's computationally feasible to do that graph analysis on everyone, while listening to every call wouldn't be.