You would want to now setup voicemail. Dial *86 using your SIP client or phone, and follow the instructions. I will personally keep this simple. My announcement is “You’ve reached Frank Pilone. Leave a message”. You can choose to be more elaborate, but I don’t see much of a point in doing this.
Make sure you follow the instruction provided to you by the interactive voicemail system. It has been ages since I needed to log into the system, so I will not be able to walk you through to what you would need to do. If you ever set up voicemail for your cellular phone, I will imagine that this should be very similar.