Hi,
i can confirm that GS290 works fine with VoLTE using e.os 1.9
Maybe you hvae adjust some things, but it works here.
VoWifi is not tested, because my local Wifi does not support VoWifi. If i can find a suitable Wifi, i will test it.
See other post: Gigaset GS290 VoLTE - /e/OS Devices / Gigaset - /e/OS community