iPhone missing and delayed voicemail notification


When I miss a call and someone leaves a voicemail, I get the missed call notification, but no voicemail notification. I don't know if I have a voicemail unless I call *86. Then, missing voicemails sometimes show up, sometimes weeks later. This happens on an iPhone 6. Something to note: It's also happening to my mother, so has an iPhone 6. And I got a replacement iPhone 6, and it still happens on the new phone. I missed job interviews and doctor calls until I realized what was happening. Can someone help? I can't seem to find a solution in the boards. Thanks.