Can I start to build and test my application during the carrier approval process?

Yes, and in fact, we encourage you to do just that. Though you are unable to use your short code while it is undergoing the carrier review process, Twilio short codes use the same REST API as our standard 10-digit phone numbers, so you can build and test your application using a regular phone number. Once your short code is live, you only need to swap out the phone number for your short code in your application. Purchase a 10-digit phone number here to start building and testing your application right away.