Check *#*#4636#*#*
To Swap VoLTE Support use *#*#86583#*#*
Most of the O2 Sub-Carriers are supporting VoLTE and VoWifi, but perhaps you are using the old eplus Network.
SipGate for example is a O2 Sub-Carrier, but they are not supporting VoLTE and VoWifi.
I dont know if eplus is supporting...