It was useless if your language isn't supported, though. For many, T9 was no longer relevant by the time their language got support.

I still miss it to this day, for basic texting it was the superior system.

I think on a PC keyboard you can simulate this by holding ALT and typing 114 on the numeric keypad

I wonder if you can change the rotary phone to octal? that would be great to boot a pdp 11/34.

I'm guessing it works out how many digits to expect for each codepoint by looking at the first digit (i.e. if it starts with a 1 then it has to be 3 digits, otherwise it is only 2).

Does this mean it's impossible to type a newline character, or a backspace, for example?

A few decades ago, you could make ASR-33 teletype printer with an acoustic modem print…something…by whistling into the modem. Why go mechanical when you can be artisanal/organic?

No idea, I didn't build the thing, just suggesting how it could work. If you want to cover [extended] ASCII, then every first digit could indicate a one or two digit code, one and two also a three digit code. So one possibility would be to have three digits for first digits zero, one, and two and otherwise two digits. All single digit ASCII codes require two leading zeros and all two digits ASCII codes starting with one or two require one leading zero. You could of course also make everything three digits with leading zeros as needed, would be easier at the cost of more dialing. Or use a timeout, if no new digits arrives within a specific time, then a new character starts.

You both are 100% correct! Originally I was going to have a wait (say, no new digits after 3 seconds, mark the ascii character), but I figured just scrapping everything after 20 as an immediate character and anything under as a 3 digit one, it made typing a lot faster.