My carrier (hollandsnieuwe, The Netherlands) supports VoLTE (dialer code *#*#4636#*#* -> Phone1 Info shows enabled “VoLTE Provisioned”).
My carrier has recently dropped 3G support. Thus, when I need to use mobile data (not WiFi) during a phone call, I need VoLTE to work.
Currently, my FP3 seems to drop the mobile data connection during calls started on 4G. I would expect that, with the FP3 supporting VoLTE, a 4G call would keep the mobile data connection.