Hello, frida116. Thank you for posting your question to the Apple Support Communities. We understand your situation and how you have been unable to complete FaceTime calls using a Wi-Fi connection with this iPhone. We’re happy to go over the available options with you.
To start, we’d like to be sure that you are able to complete each troubleshooting step suggested in the following Apple Support article:
If you can't make a FaceTime call on your iPhone, iPad, or iPod touch - Apple Support
If you complete the steps in this article but continue to notice the same issue, we can help you more. To continue, can you tell us if you notice any similar problems when making FaceTime audio calls?
We look forward to helping you find a solution.
Enjoy!