Can not receive Voicemails

Since I updated to Oreo I can't receive Voicemails and I have no idea where to start to fix it so I would ask for your assistance. Thanks

Have you already tried setting up your voicemail again in the Phone app's settings?

I'm not quite sure what you mean when you say 'can't RECEIVE voicemail'. Do you mean that:
- no one can leave VM for you?
- you can't find where to get VM that people have left for you?
- other?

You'd normally find VM in the Dialer application (fourth tab from the left). If that tab isn't there at all you may want to consider a Factory Reset. For other issues you may want to contact your carrier.