Only one Voicemail Pro client can be connected to a voicemail server at any time. To prevent an idle client session from blocking the server, the server's Client/Server Connection Timeout setting is used to disconnect the idle client session. By default, the timeout is set to 5 minutes. See 'General' System Preferences.

If your client session has timed out, the client will prompt you whether to re-establish the session or close. You are then able to continue working in offline mode or to close the client.