You have to set under Settings>Network & Internet>Mobile Networks an option called Allow 2G service.

3G towers have been decommissioned all over the nation so phone can't use that for cellular calls anymore. The phone has to fall back to 2G because Maxis are being anti-consumer by refusing the to provide VoLTE provisioning for LG phones. I have LG V35 (GoogleFi Unlocked) and LG V60 (AT&T unlocked) both can't get VoLTE to work no matter what the ROM or variant it is. The denial of provisioning is by the carrier, in this case Maxis.

I am going to be hounding their Network/Technical dept soon to provide support, in the mean time I've lodge a report with skmm over Maxis's CS asking me to buy a brand new VoLTE supported phone instead through their market place, blatant anti-consumer tactic to get new phone sales.