There was a network failure attempting to connect to this URL. Twilio will try for 10 seconds to establish a TCP connection to your URL (two 5-second attempts). Twilio will wait for a total of 15 seconds to receive an HTTP response. This includes the TCP connection time, so if it took six seconds to establish a TCP connection, your server would have 9 seconds to deliver an HTTP response. If either of those timers expire, we fail the request, fire a notification, and try your fallback URL, if one is specified.
Possible Causes
Network outages between Twilio and your web server.
URLs that reference private IP addresses or localhost.
Twilio's TCP timeout is 10 seconds. If you see an error in Request Inspector after more than 10000 milliseconds, Twilio could locate your server, but could not establish a TCP connection with it.
Twilio's HTTP timeout is 15 seconds. If you see an error in Request Inspector returned after more than 15000 milliseconds, Twilio was able to establish a TCP connection with your server, but the HTTP response from the server failed.
Possible Solutions
Verify the URL provided is accessible to the public internet.
Verify network connectivity to your web server.
Check your TCP logs to verify inbound connection attempts are successful.
Check HTTP server logs to verify a response was generated and returned within the timeout.
Check your edge element logs to verify responses are making it out of your infrastructure.