Top fact that's why 999 is 999. It was the least likely voltage to be miss understood.
Yup
Not so much the voltage but the pulse, they knew they needed the emergency number to be one you could dial in the dark or by touch if needed, and they knew that the obvious option of 111 would be triggered regularly by the most common line faults, so they went with the other end of the dial at 999 which took longer to ring but was virtually impossible to be replicated by a fault.
The US did 911 which was a better than 111, and faster to dial than 999 but it was also harder to do in the dark as you had to find the correct places twice (999 and 111 you could let your finger track the dial as it rotated back so only had to find it once).
There was one I used to watch called A1 Telephone repairs or something along those lines.
All these old phones will be retired once the landlines all change over from analog. I suspect you'll be able to get some kinda converter for them in the future made by old telephone hobbyists.
IIRC you can already get pulse to tone dial converters, as some phones from about 10 years either side of the changeover had the option to be set to one or the other and IIRC they continued to be produced for a fairly long time because of smaller legacy building exchanges (PBX?) and some countries didn't swap over for a long time so it was cheaper to make the handsets that worked with both options.
I think there are already some hobbiest kits to convert even the 1920's era phones to be IP phones by basically having a circuit that listens to the pulses then sends them through an IP adaptor that is commercially available (I've got a feeling it's done with something like a pizero or similar doing the interfacing). I know I've seen people who have done it to make a "mobile" compatible antique set.