If your parents want to keep their current landline, the OBi212 would be necessary to have. If not, like drgeoff indicated, an OBi200 or 202 is sufficient.
The most persuasive reason to keep an real landline for elderly people is for 911 purposes and ease of use. Even if elderly parents (who are not tech savvy) have e-911 with their OBi device, if power and internet goes down, they still won't be able to use their e-911. So think carefully before you transition elderly parents and especially if they are not tech savvy, out of a landline.
However, if they are minimally tech savvy and are able to do so under stress (because a 911 moment is a stressful moment), they can fall back on cell phones even if power and internet are down.
Many residential homes' phone wiring is daisy-chained (an activated line allows phones to be plugged in and shared regardless of room).
If they don't need the telco's landline, you can unplug the telephone company's RJ-11 cord at the demarcation box (though most recommend that the ends be taped or cut off so it won't accidentally be plugged in and emit electricity in the daisy-chained circuit to prevent the frying of the OBi device when you connect the OBi to the RJ-11 wall jack.). See the first attached picture for precautionary steps of unplugging and taping or cutting that cord so it won't be accidentally plugged back in.
To activate the OBi phone line into the house's daisy-chained phone circuit, you make a wire connection from an active "Phone" port on the OBi to any one of the daisy-chained RJ-11 wall jack of their house (See second attached picture.). From there they should be able to plug in any phone jack (and accompanying power adapter--because most phones need power to work) in their house and share the same line on the OBi device.