This tutorial describes how to integrate Twilio's VoIP service with 3CX, a free IP Private Branch Exchange (PBX) system. The integration allows users to make and receive phone calls using their existing VoIP phones or mobile devices, while also providing features such as caller ID, call routing, and access control. To accomplish this, the tutorial uses Python and the Twilio library to automate tasks, including updating Twilio's IP Access Control List when the user's external IP address changes. The integration is completed by configuring 3CX to connect with Twilio's SIP Trunking solution, which provides a robust and flexible connection to the Public Switched Telephone Network (PSTN).