What's wrong with case insensitive letters?
US-ABC-900-900 can be a beautiful phone number and global. 165,216,101,262,848 numbers per country.
US-ABCD-900E-900R in case we run out of numbers again.
US-ABC-900-900 can be a beautiful phone number and global. 165,216,101,262,848 numbers per country.
US-ABCD-900E-900R in case we run out of numbers again.
I strongly suspect that most countries (and certainly a majority of the world population) use a super-set of the English alphabet though.