I've always been fascinated with telephone systems, so your comments are interesting. In fact, some years ago I worked for British Telecom. That was just after privatization in about 1982; prior to that our telephone network was state-owned and run by our GPO (General Post Office), much like the PTTs you have in many parts of Continental Europe.
Digital exchanges are now installed in all but the most remote parts of the U.K. Tone dialing started to be introduced in the 1980s; the DTMF tones are the same as those used in the U.S. and worldwide, although our supervisory tones (ring, busy, etc.) are different. All lines will still accept pulse dial.
We also had party lines here, but usually with only two subscribers per line. Most, if not all, have now been phased out.
Distinction between the two parties was by simple wiring changes to the phones at each house. In British terminology the "tip" connection (+) is the A-wire and the "ring" connection (-) is the B-wire. For a normal line the bell is wired with its series capacitor across the line.
For a party line however, a ground rod was installed at each house. The bell on one phone (called the X subscriber) was then wired B-wire to ground, and that on the other (the Y subscribe) from A-wire to ground. That way the exchange could ring either bell separately.
The "Call" button, used to obtain dial tone, was wired to interrupt the normal loop and ground the B-wire on one phone (x), the A-wire on the other (Y).
In practice, the wiring in the telephone at both subscribers was identical, but the A and B wires would be swapped over at the junction box of the Y subscriber. Party-line phones were always hard-wired.
The colors used for the phone cords then were: red (B), white (A), blue, and green. Blue was normally used between extension phones to prevent bell-tinkling. Green was usually the bell return -- grounded on party lines, strapped to white at the junction box on normal lines.
The same colors are used on modern phones with modular-style jacks (not the same as U.S. types), but the bell wiring is different. Fixed wiring colors are different again.
I know some rural areas of the U.S.A. used to have party lines with 3 or more subscribers. I believe they added frequency-selective bells for ringing and outgoing calls were identified by the phone placing different values of resistance to ground.
Maybe the Austrian system used some combinaton of these methods?
