Try not using the VOIPO config and entering your information manually. I believe the standard Obi config automatically enables MWI (message waiting indicator). Otherwise you need to go to Obi Expert config -> Voice Service ->SP1 (assuming your device is on SP1) -> Calling Features -> Enable MWIEnable and X_VMWIEnable. To enable them you need to remove the check marks already there in order to make changes. If they are already enabled then you may have other issues.
Good luck!