If you already have token generation and validation logic and would like to keep those systems in place, you can do so. We have a feature where you can submit your code to us and utilize our pre-screened message templates and localizations for both text and voice.
If you're using custom verification codes you must also provide feedback that lets us know whether or not the user verified the code. This allows us to proactively monitor our global routing and stay operational.
Contact Twilio Sales and we'll help you enable this option.