User Cannot Dial Out - Error: "Failed to start call"

Overview

You are unable to make outbound calls and getting  the error:
Failed to start call

Information

• If you are using Computer-based calling (WebRTC), make sure that microphone is enabled and that Chrome has permission to access it, which is required for computer-based calling. For more information, refer to the article Use your camera & microphone.
  
  If the issue persists, change the calling preference to Phone-based calling as a workaround, also, make sure to specify your current agent number.
  
  
  
  
• If you are using Phone-based calling, your agent number may not be working (cannot accept calls) - try to make a call from your cell-phone to your agent number or change your agent number to another number (e.g., your cell) and try to make a call from Playbooks. If the number assigned to you (agent number) is not working please reach out to your Manager or Admin who handles configuration of your work phone application.