13110: Annotate: Annotate must contain one valid nested element

Twilio returns this error when you send Voice TwiML with an <Annotate> element that does not contain a valid child element. Twilio only allows BillingReferenceTag inside <Annotate>, so an empty <Annotate> element or one that contains a different child element triggers this error.

• <Annotate> is empty and does not include a BillingReferenceTag child.
• <Annotate> contains a nested element other than BillingReferenceTag. Twilio only allows BillingReferenceTag inside <Annotate>.
• The nested element name is misspelled or uses the wrong case. TwiML element names are case-sensitive.

• Add a valid BillingReferenceTag child inside <Annotate>. For example, use <Annotate><BillingReferenceTag>order-123</BillingReferenceTag></Annotate>.
• Remove any unsupported child elements from <Annotate> and keep only BillingReferenceTag.
• Verify that the child element name and casing exactly match BillingReferenceTag.
• Keep the BillingReferenceTag value at 128 characters or fewer to avoid error 13120.

• TwiML for Programmable Voice
• Call Annotation Resource
• Debugging your Twilio application