Thank you for the link! It is a bit different in my case: VoLTE is activated and can not be toggled off.
It seems that not only Vodafone is affected on the FP3, but also other providers as mentioned in this thread.
I also did a Google search with “android volte toggle missing” and there are a lot of results - about different vendors and different networks in any country. I was not able to find a solution.
Let’s see what support says about this issue.