You attempted to initiate an outbound phone call, but the 'From' number you specified is not a verified number for your account. In order to use a phone number as the Caller ID on outgoing calls, you must first verify your ownership of that phone number.
You can verify phone numbers from the phone number verification section of your account portal.
If you received this error while trying to authenticate with your Test
Credentials, you probably tried to make a test call with a
From number from
your live account. The only number that can be used to queue
calls with your Test Credentials is
+15005550006. For more information,
read our documentation on the
From number for making calls with your test