For billing, mainly.
The phone number in 3GPP protocols is the IMSI = international mobile subscriber identity, which identifies a user on the phone network. You swap out your SIM card, and you swap out your IMSI. You roam abroad, and the foreign network knows who to send the bill to.
The alternative would be to keep an international database of IMEIs (device IDs) like CDMA networks did. Using SIM cards with IMSIs ended up being more consumer-friendly.
https://www.theverge.com/a/luka-artificial-intelligence-memo...
Or using the PGP wordlist, "bookshelf amusement Vulcan phonetic crackdown alkali merit candidate aardvark Atlantic"
So obviously that's not going to work! But IPv6 isn't meant to be a static identifier. We have DNS for that.
I do think there's a future where the concept of "phone number" no longer exists. But this isn't it.
EDIT: Made it clear I agree with lemcoe9 that this doesn't work.
> "bookshelf amusement Vulcan phonetic crackdown alkali merit candidate aardvark Atlantic"
Make some chinese, japanese, russians to pronounce this on daily base.
EDIT: __aardvark__ had to google this one.