To substantiate my claim that if you have an A-GPS cell phone turned on it can be tracked here is a recent article on the subject that sheds some more light on it. The point being that you don't need to be on a call for that.
If you try to add things up you will notice that something is missing: remember the case when Sprint didn't want to give the location of a cell phone sitting in a hijacked car with a 10-month old baby inside.
Then read the N.Y. Judge rulling about why he granted permission for cell tracking:
"Gorenstein [...] said that because the cell phone user's location is only available to police when a call is in progress, and because the location information is only a rough estimate, such tracking is permissible under the Fourth Amendment."If your provider is selling, sharing, opening up, doing whatever it isn't saying with its PDE (Position Determining Entity) data, the server that keeps the tabs on your phone, it is a good time to ask about it.