It's amusing that the Government calls what they're doing with Google a "pen register". They usually do that to squeeze through the small exception in the Fourth Amendment created by Smith v. Maryland.

The pen register shown in that Wikipedia article is mine. To use it, you need physical access to a phone line, probably at the central office. You put in a blank spool of paper tape, add stamp pad ink to the ink roller, and wind it up with a big brass key. When it detects a dial pulse, the clockwork mechanism starts the tape moving, and each dial pulse produces a dash on the tape. There's a mechanical idle timer which stops tape movement after a few seconds of idle time. I built a box with a phone dial to run that brass device as a demo.

That's what law enforcement had to use in the days of electromagnetic central offices. Only long distance calls were logged. Local calls were, at most, counted. That's why the Supreme Court decision refers to a pen register as an "extremely limited" device.