What's inside Rotary Electromechanical vs Pulse/Tone Phones? -- The Electronics Inside 52

Indeed. Looks like you have the 'special edition'. 

... great book!

Another great project, this time from Enrico, which relates to rotary phones is 'The Pi Rotary':

/challenges-projects/project14/acoustics/b/blog/posts/the-pi-rotary

It also is featured in his book 'Vintage Upcycling'.

That's a good point. It feels a little "Apple using Lightning" to me?

If you want to keep it functional and iconic then converting it to work with VOIP or mobile technology is possibly about all you can really do with it. Perhaps you could get away with swapping the central printed label to convert it to circular E Ink or something along those lines.
https://shopkits.eink.com/wp-content/uploads/sites/29/2016/09/Hulk1.1_Panel_new20170418-2.jpg

If you want to let the creative juices flow though, then I think you could perhaps take a look at the general form e.g. the curves and reuse them to create a different 3D printed housing. Perhaps stretch it width-wise to accommodate a 7" LCD touch screen alongside the original dialler.

For me, I think the tactile feel of the old dialer is something that has been lost with the modern UI, and there was also something about the form of that handset with its annoying coiled cable. So nothing stopping you from reusing those basic parts within a complete new design.

I would guess it was to dissuade you from plugging non BABT approved equipment into the BT phone network which at the time would have been considered illegal.

The 'new plan' appears to have been introduced to allow the user to purchase and to plug their own phone into the telephone network without the need for a BT engineer present. A BABT approved phone (with green circle sticker) would have been fitted with the new BS 6312 plug as standard.

I had no idea that railways and motorway signs used it! Although I guess it's the biggest network with the wisest transmission makes sense?

I read a few years ago about ERTMS (The European Railway Traffic Management System), that was likely to be adopted in the UK, so it makes sense that they were looking for a long term replacement system to depreciate their dependance on PSTN?!

I really love ACAs - I think I'm just looking for an excuse to buy some to play with!

But I still have never understood why they didn't use the world wide standard of the RJ10 (or 14 depending on how many cores were actually needed). The master needs 2 cores from the exchange, internal wiring needed 3 cores to each socket. But each outlet to the device only used 2 (to the best of my knowledge)?

That was a great project from DJ! What would you make it do as a desktop phone, just a voip handset?