In the account settings of the console, one can find his test credentials to try out certain parts of the REST APIs without actually charging the account. So, the problems with test credentials not working may be that you do not receive calls or messages when using them, or your Twilio number is not working as the "From" parameter.
The test credentials are only there to pretend and not send messages or make a call. So, if you want to perform end-to-end testing without any charges, you can use the trial credit from trial mode by creating a separate Twilio project.
On the other hand, in the production project, the test credentials do not possess access to valid "From" numbers, so you can only use the magic number +15005550006 as the "From" number.
Read more about test credentials and the magic number here.
View all errors