21712: Phone Number or Short Code is associated with another Messaging Service.

Twilio returns this error when you try to add a phone number or short code to a Messaging Service and that sender is already associated with another Messaging Service. A sender can belong to only one Messaging Service at a time. Remove it from the existing Service before you add it to the new one. Removing it from a Service does not release the number from your account.

• The phone number or short code is already attached to a different Messaging Service.
• The sender is still present in the existing Service's PhoneNumbers or ShortCodes subresource when you try to add it elsewhere.

• Remove the phone number from the existing Messaging Service with the PhoneNumbers subresource, or remove the short code with the ShortCodes subresource, before you add it to the target Service.
• If you are managing the sender in the Twilio Console, remove it from the existing Service's Sender Pool and then add it to the target Service.
• If you need to fully disassociate the sender from your account, release the phone number or cancel the short code from your Account. Removing it from the Service alone does not release it.

• Services PhoneNumbers subresource
• ShortCodes subresource
• Messaging Services