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
user21
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The issue has solved in iOS 15 beta 4.
Okay, I posted a feedback with my minimal project to Apple. (FB9345108)
Thank you.