When I press the phone icon I get a blank screen and cannot check voicemail

I have recently changed from an Android phone to the iPhone SE, iOS 12.0.1. I set up the phone using lots of help from computer videos and the downloaded manual. Things were working well until I suddenly could not see the voicemail icon after receiving a call. When I tap the phone icon I see only a blank white screen even though the icon says I have 3 calls. I have tried rebooting the phone as suggested several times with no change. I'm afraid to reset the phone for fear of losing all my information. Another solution suggested I change the sim card even though it's only been in the phone a few weeks. I understand this has been a problem for others and any other suggestions would be appreciated.