What are you talking about? The symbol table has 100 symbols. A phoneme is a symbol, not a letter. Just because a symbol comprises 2 ascii letters does not make it base10. That's like saying 1 and 0 are symbols in Manchester code.
You could (almost) easily replace every symbol with a single unicode rune from an abugida like katakana/hiragana (you'd need to pull from several langs as japanese famously lacks distinction between La-li-lu-le-lo and Ra-ri-ru-re-ro (らりるれろ) but there's no reason why you couldn't encode one-rune-per-phoneme.
You could (almost) easily replace every symbol with a single unicode rune from an abugida like katakana/hiragana (you'd need to pull from several langs as japanese famously lacks distinction between La-li-lu-le-lo and Ra-ri-ru-re-ro (らりるれろ) but there's no reason why you couldn't encode one-rune-per-phoneme.
https://en.m.wikipedia.org/wiki/Manchester_code
https://metalgear.fandom.com/wiki/The_Patriots