There seems to be a general issue with how the S9 is handling notifications. I can't get notification badges working on any third party apps: Facebook, FB Messenger, Words With Friends, etc. and am having the same problem you are with missed calls and voicemail. Notifications are just DEAD for those apps. Chatted with Samsung Tech Support...no solution. Others are having similar problems and are posting about it on Samsung's Support site, Xda Developers forums and other sites. Hopefully there will be a solution soon.