I'd like to add net connection to a room that has existing telephone wires that
are not in use.

The wires (nine in all) go inside teh wall. They have not been used for years.
They were probably used for an in-house intercom (12V).

Downstairs there is a room with a lot of telephone wires coming into the room.
Many look like they had been used with the same intercom as upstairs.

How do I test which of the wires connect to the room upstairs.