Source for phone number length and memory?

In the [Types of Memory section](https://carpentries.github.io/instructor-training/05-memory.html#types-of-memory), it is noted that limits of memory are why phone numbers are 7-8 digits long. No source is listed. An old [GH issue asked about this](https://github.com/carpentries/instructor-training/issues/443) and a response referenced a [Quora answer](https://www.quora.com/Why-did-Bell-Labs-create-phone-numbers-of-7-digits-10-digits) as a source (but that itself is not sourced). Currently, [many countries have phone numbers of 10+ digits](https://en.wikipedia.org/wiki/National_conventions_for_writing_telephone_numbers).

Can we actually claim that limits of working memory are why phone number lengths are what they are, or is this a happy coincidence?

Start with the Types of Memory section in 05-memory.md, then review issue 443 and the linked Quora and Wikipedia material. Establish whether the phone-number and working-memory claim is supported; done means correcting or removing the claim and adding a reliable source or explanation.

Written by the indexing model from the issue text.