I never recommend that people set up Google Voice for their parents, especially if their parents are elderly and/or living alone. Speaking as a baby boomer myself, and as someone with experience living with elderly seniors, when essential things don't work as they should, it causes stress, frustration, fear and anxiety. The ability to work with someone over the phone to troubleshoot problems lessens. Also keep in mind, Google Voice, by itself, cannot call 911, and it requires provisioning a separate 911 service for the OBiTALK device.
So, that, plus some of the recent issues caused by this year's major changes to the service, makes it a poor solution.
A better approach is to just port your mom's land line number to a mobile phone carrier with an unlimited talk/text plan. Those plans are usually a lot cheaper than the land line service being eliminated. There's no need to pay for a plan with lots of data.