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.
> 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.