Consent can also be given on the sign-in page: Forgot password? texts a 6-digit reset code, and requires the same unchecked-by-default consent box before the code is sent. Consent is recorded once the code is entered, which confirms the number belongs to the provider.
Providers can opt out at any time by replying STOP, or by clicking Revoke on the same settings page. Replying HELP returns support contact information.
The opt-in form as it appears to signed-in providers (preview, not interactive):
Questions: khaled.fernainy@gmail.com · 404-824-8525