Xiamoi mi A2 lite

[Android Central Question]

Hi guys purchased xiamoi mi A2 lite global to use in UK but cannot get volte to work I'm on three network bands 800 and 1800 4g working fine, I've checked in settings but there's no switch to toggle on so checked to see if provisioned which is greyed out and can't be switched on. Hope you guys can help, TIA Andy