Pen recorders and phone records

I have read that if the police want to know who a person communicates with on the telephone, they can arrange to have a "pen recorder" put on their phone line. This will record all phone numbers called from that line. Supposedly the legal barriers to this type of surveillance are much less than for a phone tap. I am confused about the necessity for this if the phone companies routinely record this information anyway. Is this just an archaic and obsolete terminology, and what really happens is that the phone company will give already-existing phone records to authorized officials? Thanks - Hal