@oliveoil: Do you have Do Not Disturb active on the device? To check, head to Settings > Notifications > Do Not Disturb, and toggle this option off. I'd also recommend checking your Call Forwarding and Call Barring settings by opening the Phone app > Tap the 3 dots in the top right > Settings > Supplementary Services > Call Barring > Voice Call > Make sure all options here are toggled off, specifically 'All Incoming Calls' > Back > Call forwarding > Voice Call > Disable any active options here. I hope this helps.
