The purposes of phone numbers:
1. Verify you are a not a bot: no need to store anything except TRUE once verified.
2. 2FA - well use something better than SMS, but if you must, store the hash, and make me enter my number for the 2FA each time. Compare with hash and then send SMS.
- Account search during password recovery (lets users search for their account by phone number): https://twitter.com/account/begin_password_reset
- User discoverability and account recommendations (users who upload their address books can find others by phone number, users who share their number can be found by others): https://help.twitter.com/en/using-twitter/account-suggestion...
Hashing numbers has other implications, like support impact (some folks don’t know their own phone number), preventing the ability to offer SMS updates in countries that need it (or to reactivate that feature in national emergencies for countries that SMS support was pulled from), as well as making potential marketing, data mining, satisfying legal requests, and future feature development harder.
So your suggestion is a good one for a privacy-conscious service that doesn’t already depend on (or that is unwilling to relinquish) unhashed numbers, but it probably isn’t in the nature of twitter to seek to protect user data at the expense of existing or future features, even after leaks like this.