Return the one account-level URL a customer copies into Twilio’s Messaging config.
The route is visible only after the BYO account is verified. It does not try to make an
unverified credential or a platform-managed account look messaging-ready, and it never
serializes the credential reference or either credential field.
The exact signed Twilio Messaging URL for one eligible BYO account.
This is a configuration address, not a credential: Twilio still proves every request
against the account’s vaulted auth token, and the destination number is signed and
routed again on receipt.