Register an iMessage sender

Registers a provider-provisioned iMessage sender (a phone number or an email handle) that YOU already own on a profile, creating an imessage account that sends and receives through the inbox conversation endpoints. To have Zernio order a new sender for you, use POST /v1/imessage/senders/order instead. Registration attaches the monthly sender fee (billed while active) and requires a payment method (402 without one). One sender per profile: re-registering the SAME handle refreshes it; a different handle returns 409 until the existing sender is canceled.