@claude0001, thanks a lot for your answer. I was wondering from where you saw I'm located in Germany (hopefully it's not that clear from my English skills 😀 -> saw now it could be the app names), but yes in general it's true. Anyhow, I just started using my Pro1-X and only used it in Spain so far - but all mobile phones participating in my calls so far were German phones. I'll try your solution later on today. I'm curious, hopefully it works 🙂
I've tested it. There is no option "VoLTE", but one called "4G-Anrufe" (which means "4G-Calls"). I've disabled it. 2 of 2 calls worked well so f