You kinda missed one of the major reasons of why phreakers built the beigeboxes. We used them to find lines that were connected to the phone company switches but not hooked up to any subscriber, so that we could make expensive calls that nobody would have to pay for. For example, long-distance or Internet dialup.

The beigebox was basically just a DIY Lineman’s Handset (LMHS), a common tool used by telco workers for troubleshooting when doing field work.