Have you tried restarting the phone? Sometimes that is all it needs. Also, you may want to check to see if there are any updates to your phones firmware. Either/both of these may resolve the problem.
Also, if you have an app, like the "Voicemail" one on my phone that I so hate, it can cause this problem. This isn't regular voicemail. It is an app that converts them to texts. It tends to turn off sounds and not notify you of missed calls.
If it persists after that, I would contact your service provider/carrier and make sure there is nothing wrong on their end.
Worst case, you may have to a "Hard Reset" on the phone to fix the issue, but I would save that for last as it wipes all your data on the phone when it does the reset.