Twilio makes HTTP requests to your server to fetch your app's TwiML instructions. Some users prefer to know which IP address the request from Twilio is coming from in order to open up specific ports in a firewall. However, due to the fluid nature of our cloud architecture, we don't have a set range of IPs that requests are sent from or know in advance what they will be.
Because Twilio's requests will be coming from different IP addresses, we instead recommend that you validate that a request came from Twilio by other means. Please see our documentation on securing your application for more details.
If the inability to have a request come from a static IP address is a serious concern, please contact our sales department to discuss other options which might be available.