Why Did Sony Stop Including VoLTE & HD Voice In Their Xperia Line??!

Previous models like the Z2 and Z3 Xperia phones had both VoLTE & HD Voice included, even in the international versions. But since the Z5 and even with the upcoming X series phones, these features were removed. Does ANYONE know why Sony did this?? I have a couple ideas, but I'd like to know if anyone knows anything more concrete. Any information would be great!!

Thanks!

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