Why is FaceTime not working on my child's iPhone 12?

I recently upgraded my iPhone and gave my old iPhone 12 to my 9 year old daughter. An apple id has been created for her. I have set facetime to be an always allowed apps under screen time and have enabled Facetime under "settings" in her iPhone.

Although there are ticks next to her phone number and email address in the Facetime setting under "You can be reached by Facetime at", Facetime doesn't actually work.

In the contacts app in her phone, no facetime options can be found in any of her contacts. In my phone, there is no option to facetime her in the contacts app. When I tried to facetime her using the icon at the top right corner in the message app, her phone didn't ring and my phone said she is unavailable after ringing for a while.

I have already updated her phone to latest IOS. I have restarted her phone, tried the disable - enable trick but none of these worked. I couldn't find a solution online so I came here for help. Thanks.

[Re-Titled by Moderator]