Sony Xperia V IV compatibility

I've just switched over to Smarty from BT mobile. Using an unlocked Sony Xperia V IV .

It seems this phone is not compatible with Smarty WiFi calling or VoLTE ?

Additionally I am unable to get any incoming voice calls - they all go direct to vmail. I can make outgoing voice calls ok and have a good signal -105dBm and 4G.