To test your configuration application is passing the OMTP configuration correctly you can test your code with: A SIM containing a valid certificate signature A device running Android 6.0 with an unmodified version of the AOSP phone framework
A business’ voicemail greeting is often the first point of contact with a business’ customers. Getting this greeting right can make a business sound professional and trustworthy. A badly-done greeting, on the other hand, can confuse and infuriate customers over the phone.

Once you have your message, you need to actually record it. The exact process varies depending on whether you’re using a cell phone or office phone, but here’s the basic process: Press the voicemail button, or press and hold 1 on most cell phones. Enter your password. Record your message. Listen to the message you just recorded. Follow the prompts to save your message. Following Up on Voicemail Messages
Creating a professional voicemail greeting isn’t complicated, but you need to keep a few things in mind to ensure success. The following tips will help: Be friendly and welcoming - let your company's personality shine!; Have a clear voice, speak at a slow to moderate pace, minimize background noise; Ensure the greeting is human and approachable; Keep the greeting short and informative; Ensure the greeting doesn’t sound robotic or unnatural; Show your gratitude for the call by saying thank you; Manage expectations by clearly stating when the client can expect a callback.