Twilio provides test credentials for you to exercise parts of the REST API without charging your project.  For where to find your test credentials see Where are my Twilio Test Credentials?

Please note: Twilio test credentials will not work with Twilio CLI. 

Below are a few common issues we often hear about regarding exercising test credentials:

I never received any messages or calls when using test credentials.

Test credentials only provide a way for you to "pretend" sending a message or making a phone call. Twilio will not charge your project, nor will the actions be performed. You would rely on the response and error codes Twilio returns to verify if it would be a valid request to Twilio when initiating the request using your live credentials. The SMS SIDs or call SIDs Twilio returns when using test credentials are fake SIDs and they cannot be found in the logs section. If you are looking for end-to-end testing without being charged, we recommend that you create a separate Twilio project and stay in the trial mode to take advantage of the trial credits with the Twilio Free Trial Limitations.

My Twilio number doesn't work as the From parameter.

Your test credentials don't have access to any valid 'From' phone numbers on your production project. Therefore the only phone number you should use as a 'From' number is the magic number listed in the documentation.