A voip phone line, especially via fiber to the home, has far more in common with a traditional copper-based line than it does with a cell phone. It is really just an upgraded landline phone.
I recently changed from copper to fiber in rural central Tennessee, and it's been a godsend. The fiber, obviously, needs no power. In an outage, I have phone and internet as long as the UPS that powers the adapter and router lasts (easily 12hr if nothing else is connected). In contrast, my copper line would go dead within 20 minutes of an outage, b/c Frontier refuses to invest in battery backups for the boxes along my road. My copper line would also have static and noise whenever there was a hard rain.
Cell service at my home is better than it was years ago, but I still often need to go outside to get a bar. Dropping a landline isn't a good alternative for me.
This came about b/c a local teleco received a federal "rural broadband initiative" grant. I'm now paying about $80/month for my phone and advertised 500 MB/S, instead of $130/month for the phone and advertised 5 MB/S. My actual copper speed was often below 4, and the fiber often exceeds 800. Other than being more reliable, 200 times faster, and about 40% less expensive, the new service works just like the old copper landline.