Most VOIP providers offer this at no extra charge. I'd request this be made part of the basic service.
For premium service, it would be nice to have an opportunity to call several possible alternate lines at the same time.
Ooma is great, but my current VOIP provider offers this failsafe feature. I can't move my main home phone line to Ooma, until it can safely be assured of calling my cell if for any reason the internet VOIP connection is down.