Dual VoLTE service is already enabled on the Nokia 7 plus. Did you update your phone to Android 8.1 yet?
I have checked a bunch of YouTube videos. I saw it working live on the Nokia 7 plus. Nokia 7 plus is also listed under the list of Dual VoLTE devices.
Just in case it is not yet available, it should be coming with the next software update. It got enabled on my Nokia 7 (not the 7 plus) with the 8.1 update which Nokia pushed a few days back.