"Sign in with $Clearinghouse" could bring you to a page that prompts whether you want to share a user ID or the phone number, as required, with that service.
The clearing house verifies you only once, or once a year, instead of every time. If the clearing house were to be a nonprofit, perhaps even set up by Signal themselves to spread costs with similar services, that has to be cheaper.
It also gives users confidence that only a randomized user ID was shared, so it won't be used for cross-service correlation and tracking, if the service didn't actually need your phone number but only some identifier.
This does not reduce the overall cost, it just shifts it to the clearinghouse. Who pays the clearinghouse so that they can cover their own exorbitant SMS costs?
The clearinghouse may not have the user’s most recent email address, which is common amongst non-tech people. My mom and aunts have lost many email addresses this way and forcing them to use an older email would cause many issues.
The app has to ask for email/phone to begin with (see step 1), if the email doesn't match then phone would be used as fallback, or potentially as a "Didn't Receive Code?" gesture.
