I’ve encountered certain applications in which the Enterprise Concept of Hold & Transfer is not as suitable as the old fashioned “Hold” button … where you put line #2 on hold and button #2 blinks on all phones and you go to any other phone and press button #2 to pick up the call.
Obviously it limits the incoming calls to the # of buttons on every phone and for some people it’s a step back in time but it’s also much more useful for Mom & Pop in some instances.
Switch and lamp units had analogue wiring for all the lines to all the phones, as far as I know. There was no central control. Whilst it might be possible to simulate it, it is not going to be a good fit for central control and SIP. Maybe you should look into features like parking, instead.
The best way I found to emulate the old “key system” look is to program BLF park buttons ( 71, 72…) on all of the sets. After answering the call the user can simply press one of these buttons if it’s unused and it will light up on all of sets. Anyone else can press it to pick up the call from another phone. Depending on the phone type you can program as many park buttons as you need or that can fit
It’s always a challenge trying to teach users that the old “line” system is gone and Park is now their friend. If they have phones with a lot of BLF’s available the method suggested by @rpcomm usually works pretty well and I’ve used that for a bunch of clients.
Otherwise you just have to educate them on the concept of parking vs hold/transfer.