I have four PBX extensions linked to four CO lines of 551C shoebox. Incoming calls from the true outside lines (external dial-in numbers) are assigned in PBX to these extensions. So from KSU it looks like every external line is linked to each “button”. But it is only partially true, because it is indeed linked when call arrives on the “free” button. If it arrives on a busy one, hunting group in PBX will automatically move it to the next available button. If both buttons are busy - it will move it to the third, etc. So if I want to “transfer” the call, I either do it via PBX transfer function (as described above by Silversam), or put line on hold and call via other line=extension to another phone and ask to pick the line from hold. That was the idea of typical 1A2 installations: to cover the needs in communication in the environment where neither full fledged PBX nor switchboard with attendant will be justified by the scale of tasks or to expand services of PBX to provide control over several external lines/PBX extensions.

Example: Boss’s wife calls in. A secretary takes the incoming call on external line 1. When she recognized that she needs to transfer the call to her boss, she puts wife on hold, calls boss via intercom and announces: “Your wife is on Line 1”. Alternatively, if the boss is not at his place she can announce via Public Address (=Paging,=Announcement system), linked to special extension number of intercom system: “Mr. Smith, your wife is on Line 1”. Then boss can pick up this line from any station in the office having access to Line 1. Is it pure “transfer” to the required extension? No! Does it cover 90% of communication needs in typical office? Yes! This is where 1A2 surves that “fit for purpose”.

In my case I have 7 incoming dial-in numbers, but I’ve spread them across 4 buttons of 1A2 (via assignment of incoming CO lines to the extensions of PBX connected to 1A2 line cards), and I’m fine with that because I don’t need 7 lines+Hold telephones, but I can use standard WE 2565/565 ones and a 551C KSU with 4 line cards in it.