An extender is designed to be used for making phone calls without directly billing the caller. Most mid-size and large companies that operated from multiple cities/countries used them.
Extenders connect to a PBX that is normally exposed via a toll free number (1-800), or a local number that allows the user to dial out to long distance numbers without incurring long distance connection charges.
Itβs basically how prepaid long distance cards work nowadays: You dial a number, enter a code, get a dial tone, then dial your long distance number to connect.
I grew up BBSing in Southern NH where you could not make a free call from Manchester to Nashua or Concord but it was a free call to towns in between those cities like Hooksett or Derry and a free call from those towns to adjoining cities.
One cheat we developed was a script that would make the BBS hang up, program the call forwarding, wait for a ring, then remove call forwarding. This way the caller could call a local BBS, choose a BBS in a distant city, hang up, dial again, then reach the second BBS.
It never really caught on for a few practical reasons, one was that somebody else could dial the first BBS when call forwarding was active and end up reaching the wrong BBS.
circa 1988 and we were both into serial programming. He wrote a program that would answer the phone on one line with a modem, let the caller pick a target BBS, then it would dial out on another modem to connect to the BBS. Unlike the other solution it tied up two phone lines on the relay for the whole length of the call but it never redirected people who weren't expecting to be redirected.
Extenders connect to a PBX that is normally exposed via a toll free number (1-800), or a local number that allows the user to dial out to long distance numbers without incurring long distance connection charges.
Itβs basically how prepaid long distance cards work nowadays: You dial a number, enter a code, get a dial tone, then dial your long distance number to connect.