Twilio provides you with a set of test credentials so you can exercise parts of the REST API without charging your account. You can find your credentials on the Auth Tokens page of your Console.
You use these credentials in the same way as your live credentials, with one restriction being that you cannot log in to the Twilio CLI with your test credentials. However, when you authenticate with your test credentials, we will not charge your account, update the state of your account, or connect to real phone numbers. You can now pretend to buy a phone number or send an SMS, without actually doing so.
To protect your production data, your test credentials can't interact with the data in your real account. For example, you can't use phone numbers from your real account as the 'From' number in requests made with your test credentials.