iPhone 15 running iOS 26.1: How to fix default calling app reverting to FaceTime audio?

Im iphone 15 ios 26.1...having problem with default calling app...phone keep on revert to facetime audio as default calling app even after i set it to mobile data..and its so annoying when i cant received any call on mobile data..how to fix this...

[Re-Titled by Moderator]

Original Title: Default calling app bug