Okay, I posted a feedback with my minimal project to Apple. (FB9345108) Thank you.

The issue has solved in iOS 15 beta 4.

Thank you. I tried your code already, but it doesn't work. iOS 15.4 seems never allow USSD number for security reason or something. eskimo + "1" + "@apple.com" says 'The Apple URL Scheme Reference describes this limitation and the reasons for it.' https://developer.apple.com/forums/thread/14766