I've always thought this was BS. But it was accepted in Smith vs. Maryland, which was cited as precedent here. The dissent there was that picking up the phone that Ma Bell gave you, therefore connecting to Ma Bell's network, so that when you called somebody else, Ma Bell would know that the number that Ma Bell gave you was making a call, so that Ma Bell could send you a bill from Ma Bell, was somehow "volunteering personal information" and not simply dialing the damn phone.
There was a somewhat similar case before the court within the last year or so. In that case, the question involved the application of Fourth Amendment protections to a rented car. The Supreme Court decided that the police did NOT need a warrant to search the car in that case. However, there was also another angle in that the person driving the car was not the person who rented it ... and the police had every reason to believe it had been stolen.