Proftech:
I already have Google set up for incoming and outgoing at no charge. I'll likely add Callcentric for 911 at 1.50 per month. If I some day choose to get rid of Google, I'll probably go with Callcentric for 911 and incoming/outgoing on a per minute basis. I figure that would be about 3.45 per month plus roughly a dollar an hour I'm actually on the phone. Total bill should rarely be even $5 a month.
My remaining concern is the uptime reliability of my new phone service, which has multiple fail points: Google, Obi, Callcentric, Internet provider, power outage, or the Panasonic phone itself.
I've been without Internet service for about 10 hours in the last year and without phone service for only 2 or 3 hours. It remains to be seen how much Google, Obi, and Callcentric would add to the phone downtime.
Thus the search for a last resort cheap fallback emergency method. Anything that is say $10 a month or more is pointless—I'd be better off without the Obi and just use my old service at maybe $15 or $18, with greater reliability.
SDB:
It doesn't look like T-Mobile or TracPhone have much application to my situation, particularly since as you say they can change the terms at any moment.
I may take a look around Walmart.
In the meantime, I'll keep evaluating my uptime with my current configuration and decide if I want to risk simply not having phone service from time to time. That generally wouldn't bother me, excluding police, fire, or medical emergencies.
Any further ideas welcome--particularly your personal levels of phone downtime using VoIP.