The DIY Telephone Exchange

So by now I've made a couple of projects and gained a good understanding the ways of the telephone network. Based on what I had learnt what would be nice would be to revisit the Plan 7 and think of a way to make it work properly, as it was designed to work. What I really needed was some kind of method to get connected into something that would detect the dial pules, de-code them and route calls to the desired phone on a telephone network. And the same thing but going in the other direction. Then I could use the extension phone properly in that I could dial out from it and transfer calls to it. What I really needed was a DIY telephone exchange. So really the point to this blog is that I just wanted to make a few notes about my journey from knowing nothing about electronics and telecom systems to actually building a working telephone exchange of my own.  I'm afraid it's not going to be a detailed "how to" type of thing and I'm not going to release any of my cod...

Before getting on to the discourse on the DIY Telephone Exchange, its worth a quick look at the Plan 7 system I mentioned in post 1... A long time ago the GPO used to offer a telephone system called an Extension Plan . This was a very simple sort of setup where a single "master" phone could be used by an operator to direct incoming and outgoing external calls to a one or two extension phones and as an intercom. Basically the kind of thing that would used in a simple office setup. The Plan 7 master phone has a selectable handle that allows the operator to accept incoming calls, put incoming calls on hold, route the incoming call to a maximum of two extension or to talk to directly to an extension. On the master phone and on each extension phone there is a plunger that rings either a buzzer in the master phone or a trembler bell in the extension to summon either the operator or the owner of the extension phone. When an extension is occupied on an external call a relay operates ...