But won't that fail at the same time your home internet access fails - when the ISP link goes down? I have a land line as a redundant link, for security.
An actual POTS circuit provides redundancy, but many people's landlines aren't actually POTS anymore, they're the ISP's proprietary version of this with an ATA inside the cable modem. This should have the same failure profile, while being cheaper and more fun.
Telco/ISP provided "VOIP" is often technically Voice over IP (as in the protocol), but regulations and implementation can be subtly different from VOIP (what people think of as Voice over the Internet AKA "over the top" or OTT).
ISP-provided Voice often comes with requirements for battery backup in the fiber/dsl/ata box so the user can call 911 during a power outage. There's also reporting requirements for outages that prevent a user from dialing 911.
Also, with ISP-Voice the VOIP part is considered a implementation detail so it can be given absolute QoS priority over internet traffic on a given last-mile link. This helps with being able to call 911 while someone leaves bittorrent open or whatever. OTT voip doesn't get that benefit (and that's not technically a net-neutrality violation).