Just looking at a real telephone immediately disabuses one of the idea that they are numeric or even alphanumeric. Go look at the keypad of a POTS touch-tone t

Just looking at a real telephone immediately disabuses one of the idea that they are numeric or even alphanumeric. Go look at the keypad of a POTS touch-tone telephone. You'll see 10 digits and 2 non-numeric non-alphabetic symbols. In fact, there are at least 16 possible characters at the signalling level, considering just DTMF alone, and not counting punctuation added at the human-readable level like hyphens, spaces, dots, and brackets. Then there's SIP. (-: