carriers maintain logs of where your phone is at all times by logging the towers that are servicing your phone. There are several legitime reasons to to so like analyzing network loads, quality of calls and improvements need for service. They also use that to provide customer service (some do better than others).
They also keep all your text messages, phone numbers that interact with your phone (numbers you call, numbers that call you, numbers involved in txt) and how long each call took.
That's just standard data collection.
Now the scary part. Depending on the type of investigation needed they can also track conversations involving multiple parties (you call a friend, that friend calls someone else, etc.) and your actual voice conversation can be analyzed in multiple languages. Oh yes and all of that in real time.
If you are using an electronic device today be sure you are being tracked somehow. That's just the price of modern life. Wait until your car insurance company starts to mandate you put a GPS in your car so they know how dangerous of a driver you are (speeding, where you go, what time of day, etc.) Some companies already offer a discount today if you do it voluntarily.