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?