Thinking about this strictly as a "business continuity" problem:
How often does their Comcast VoIP service go down, with their Comcast internet service still functional? It doesn't seem like that would be very often.
Let's assume that this DOES happen. Now, you want to use a different VoIP provider over the still-functional Comcast internet service. What about Google Voice? Well, as discussed in a different thread, Google, and their telco/VoIP partner(s) don't pay for premium VoIP routes, and don't seem to do a great job ensuring quality of service. Given that, any sort of modem-like device (an actual old fashioned data modem, or a fax modem, or a POS terminal, or a Medtronic device, or a security alarm) is not likely to work reliably, if at all, over GV. GV is not comparable to Comcast "Digital Voice" in quality or reliability. By contrast, Comcast uses their own, private, managed IP network for as much of the call route as possible, so they can perform QoS, route prioritization, etc. GV doesn't do this.
Now, instead of GV, you could try a pay-by-the minute SIP-VoIP provider, like
voip.ms, who offers premium routes, but you won't know how well this works without trying it.
I think a better solution, from a "business continuity" standpoint would be to upgrade their POS to a modern, TCP/IP system (something that runs on a computer over the internet, or runs on a smartphone over WiFi or 3G/4G WAN, such as Square or whatever the Intuit Square-competitive offering is called), and/or to get a battery-powered POS terminal with a cellular data link (these are popular with vendors at outdoor events).