> Service A => User: Please Enter Your Phone Number and Email
> Service A => Clearinghouse: Please verify phone number XXX wants to sign up for an account with us
> Clearinghouse => User (SMS): Please respond with the Email you used at signup to confirm you want an account with Service A
Later...
> Service B => User: Please Enter Your phone number and Email
> Service B => Clearinghouse: Please verify phone number XXX wants to sign up for an account with us
> Clearinghouse => User (Email): Please verify you want an account with Service B
Not saying it's great (providing email twice is annoying), but it's something.
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.