13337: Gather: callback must be over HTTPS when using gather with PCI compliance

This error occurs when a <Gather> action callback used in a PCI workflow is not served over https://. Twilio requires a secure webhook endpoint for this flow, and Twilio won't connect to an HTTPS URL with a self-signed certificate.

• The <Gather> action URL uses http:// instead of https://.
• The callback endpoint uses an HTTPS certificate that Twilio cannot trust, such as a self-signed certificate.

• Update the <Gather> action URL to use https:// and make sure the URL is valid.
• Use a trusted TLS certificate on the callback endpoint. Do not use a self-signed certificate.
• If you are collecting PCI-sensitive data, enable PCI Mode for the account in Voice settings before using PCI workflows.

• TwiML Voice: <Gather>
• Payment Card Industry Programmable Voice workflows
• Webhooks security