Twilio does not publish a list of IP addresses which make HTTP requests because the pool of IP addresses which we use can change frequently.
If you are interested in confirming the origin of requests coming to your server, each HTTP request contains a cryptographic signature header which can be used to validate that the request is legitimately from Twilio.
If your internal network environment requires whitelisted IPs in order to allow access, we suggest hosting a server outside your network (e.g. in a network DMZ) to proxy requests from Twilio into the internal network.
See also: All About Twilio IP Addresses