WordPress Twilio SMS Integration

I need Twilio-powered SMS notifications running smoothly on my existing WordPress site. The goal is straightforward: when a defined event occurs on the site, WordPress should call the Twilio REST API and deliver an SMS to the relevant recipient.

You will decide with me whether this is best achieved through a well-maintained plugin, custom code in a small standalone plugin, or a functions-only snippet. Whichever route we take, the end result must:
• use the latest Twilio PHP SDK (or native REST requests if you prefer)
• respect WordPress coding standards, hooks, and nonces where applicable
• include a clear settings page or filterable constants for the Twilio SID, Auth Token, and From number
• let me adjust message templates without touching core code

Please handle the full flow—API authentication, secure credential storage, callback handling for delivery status, and on-site testing—then hand over concise documentation so I can maintain or extend it later. I will supply admin access and, if needed, fresh Twilio credentials once we’re ready to wire everything together.