Hey there martininu,
Thanks for reaching out to the Apple Support Communities. We understand you're having issues with your phone not ringing when receiving a call from one of your contacts that's listed as a 'Favorite', and we'd be happy to help.
To get started, is this occurring when you have Do Not Disturb enabled? If so, let's ensure that the contact is still in your Favorites list using the steps listed in this link here: Use Contacts from the Phone app on iPhone - Apple Support
From there, ensure that 'Allow Calls From' is set to 'Favorites' in the Do Not Disturb settings as shown in this link here, and let us know if that helps: Use Do Not Disturb on your iPhone, iPad, and iPod touch - Apple Support
Take care!