Blu Kuban is not a JB build, it's ICS.
Are you saying the problem is that you're not getting voicemail notifications? If that's the case, you'll need to download the latest apk of visual voicemail. Even if you don't subscribe to it, you'll need it installed to get notifications.
EDIT : go into settings, rom updater, new. Look for the Sprint Visual Voicemail and download that. Even on Boost the Sprint Visual Voicemail is what you need to have installed.