You'd be looking at Category 1 LTE, which is LTE without the MIMO. Some Category 1 modules are starting to support VoLTE. This stuff is relatively new, which might explain the answers you've got if it has been awhile since you've asked. It's still strongly recommended that Category 1 devices use a receive diversity antenna, but they work without it. Less robust, obviously.
Ah! Cat1 modules aren’t actually shipping yet with VoLTE in volume. Manufacturers are sampling them, but as of June weren’t shipping yet. Expected by year end last I heard. Maybe you have better sourcing, but that info makes more sense now. I’d love to pick your brain more, if you’re up for it, my contact info is in my profile.