This is essentially trainspotting for telephony nerds. Most of this hobby is exploring and mapping telephone exchanges just by dialing random numbers and listening, and sometimes discovering interesting unlisted phone numbers. The jargon is mostly referring to identifying telephony switching equipment based on dialing a lot of phone numbers in a given area code then listening for the subtle audio clues that hint at the type of switching used by that exchange. Lookup Step by Step switches on YouTube for to see what old timey telephone exchange equipment was like.