Yes it is booked into 4G.
As I work for a telco I was able to contact a colleagues at Sunrise UPC who works in the mobile department and learned that they have tested the Fairphone 4 and found Issues with VoLTE, Wifi Calling and 5G. Therefore they intentionally disable those features when a Fairphone 4 logs into their mobile network.
They are in contact with the manufacturer of the Fairphone 4 to solve those issues but it has no priority as this is not a phone they sell or advertise themselves.
So let’s hope time a update FP4 firmware solves the issue.
-Benoit-