As Mike pointed out, you can do the phone#@carrierDomain trick, but not sure if that's still working. I've been doing some testing and they VM notifications to 5552128900@vtext.com (fake ph # obviously) are not going through. But, could just be my situation.
17 Sep 24: Update The phone#@carrierdomain method not longer works. I confirmed this unofficially with a resource at 8x8. This is related the same changes enacted in June of 2024 which prevented WORK desktop users from being able to send SMS unless an SMS camplaign was officially registered.
My next course of action will be to try email forwarding. Step one: Set VM notification to some business type email address. Then have that business email address kick-off a relay to the phone#@carrierdomain. I've not tested this yet.
20 Sep 24: confirmed with my customer that this method (described in the previous paragraph) is working.