Alas, you've just missed the boat for getting free Internet telephone service via the ObiHai and other devices through Google Voice. That's what I've been doing, but their free service ends this month, so I've had to sign up with another provider.
I've been very happy with the Obi110 I bought several years ago. It's given me great phone service (through Google) for very little money. Now the GV free ride is ending, I signed up with PhonePower, which I've only just started with, but looks like it'll be a good choice for our household. We picked their $35/year plan, which has 300 minutes of outbound calling per month and a penny-per-minute rate after that. More than we need, but they have a $60/year unlimited plan, too.
Assuming you have broadband Internet and a router with an open wired Ethernet port, I'd probably go with an Obi110 and pick one of their "approved service providers." Both PhonePower and Anveo offer great plans, any of which beat the pants off of local phone company offerings and cost. Your dad will get cheaper phone service with more offerings, probably won't notice any difference between traditional landlines and the Obi110 line, and can take both the ObiHai interface and his phone number with him if he ever moves elsewhere.
One last thing. You can go to, say, PhonePower and order their service with their "free" adapter, but they PhonePower charges more when you get their adapter, and their adapter is tied to their service. The ObiHai works with several service providers, and, if your provider goes away like Google Voice has or their service quality declines, it's pretty trivial to switch providers.