Sounds like unlawful surveillance to me.
Clarence Thomas of all people ruled in favor of this kind of “surveillance” in a case before the Supreme Court a few years ago. His legal reasoning was sound. Since a mobile phone customer gives ownership of the phone data (the location and browsing information, not the contents of the calls) to the mobile phone service provider under the terms of the service contract, this data isn’t covered under the Fourth Amendment protections against searches and seizures without a warrant.