Zoiper can notify you to voicemail messages left on your extension if you are not using the voicemail to email service.

Click the Envelope icon on the bottom left of the Zoiper app.

In the extension box enter *97 and click the Add button

Now when a voicemail if left the envelope will flash with the number of voicemails next to it.

Please note that when you close and re-open Zoiper the indictor will not work until a new voicemail message is left.