If you share the same wireless account and the account is in your name you may be able to track the phone usage. If you have web access to your account you may be able to access that information. If you don't share an account you should not have access.
There's no way to monitor someone else's phone without their knowledge. The only people allowed to do that are the police and the security people like FBI. They do this if they suspect the person is involved in crime or anything to jeopardise security.