The tutorial discusses how to validate E.164 formatted phone numbers using Twilio's regex pattern in Go. It covers creating a small Go-powered API that accepts requests with a phone number and returns a JSON response confirming whether the number is correctly formatted or not. The code defines two functions: `IsValidPhoneNumber()` which uses the regex pattern to determine if the phone number is formatted correctly, and `validate()` which retrieves the phone number from the request, validates it, and returns a response to the client. The tutorial also mentions that Twilio's E.164 regex does not accept valid phone numbers written in other formats and provides instructions on how to test the code with curl.