And if mobile phones were 100% guaranteed to be charged when you need them, that would be fine, but since they're not, asking a child to memorise a few digits so they're able to use someone else's phone in an emergency is perfectly reasonable redundancy.

Also, as far as I know, there wasn't ever a time when people knew how to send telegrams, but didn't know how to write letters. Indeed most people didn't know how to send telegrams with a telegraph key at all, they'd just write up their message and take it to an office to be keyed by an operator. A process which is largely identical to posting a letter.