When talking about phreaking and party lines, sometimes there's slight confusion when it comes to younger people or non-phreaks, because there's the official term Party Line, which you can read about on Wikipedia, and the more colloquial term used among phone phreaks also just regular people screwing around on the phone in the 1970s.
Party Lines are basically like conference calls and phone companies had official ways of doing them, mostly through special dial ins and things like that. Everyone would dial a special phone number and all be connected together (one manner), and you can read more about that on Wikipedia.
At other times when people referred to party lines (especially in the phreaking community) what they were referring to were things such as:
- Dialing test circuits, which would play a constant 500Hz tone, then flash the phone (fast hang up on old telephones), which would stop the tone, but keep you on the line. These old test circuits no longer exist, instead have been replaced with ring-backs.
- On many switches years ago, error messages such as a phone being disconnected would be played from one place, so if people made the same "mistake" in dialing, they would be able to speak to each other.
Among others, if you have another way this was done, please detail it in a comment and I'll add it.