Welcome to iMore! Please provide some more details so we can help. We aren't sitting in front of your iPhone so any detail you can give us will help us better help you.
Are you getting any incoming call notification on screen?
Do callers state that their calls go right to voicemail or do they hear ringing on their end first?
Have you tried the usual suspects like resetting, making sure the volume is up, making sure your phone isn't on Do Not Disturb, and making sure you're not in silent (vibrate) mode?