I didn't have a clear direction after that, so I proceeded with the works-right-now solution of Google Hangouts (with all of its associated proctology). I haven't done much else with it besides backslide a bit by getting a cheap SIM so that it would ring reliably (Hangouts is a flaky POS).
Location tracking is my biggest concern (Getting on the PSTN anonymously seems like a completely different problem, and I'm less interested in tackling it), but I don't see much way around it when you're using the cell network for backhaul. Unless you religiously shut off your wifi point, pay for a new wifi point + plan in cash every few weeks, and get enough people doing this that you can blend in.
IMHO all of these guides that talk about prepaid sims and burner phones really only work for exceptional situations where someone is willing to jump through many opsec hoops. They aren't congruent with people's standard expectations of cell phones. Any solution has to roughly work with people's expectations to be adopted, since most people only casually want to defend their privacy.
The proper privacy-preserving cell solution would use bearer tokens to pay for network access, and have no device identifier tokens. This is obviously a pipe dream. The only advance I see on the horizon is as more wifi points open up, ideally coupled with software control of your identifiable cell radio that would selectively allow tracking for checking in if it had been too long since seeing a wifi spot. Most people are probably only out of the range of wifi for an hour at most times, so if it was acceptable to delay reception of messages that long, a lot of every day privacy could be practically achieved.
If you had $100M perhaps it would be possible to start a privacy-preserving MVNO with devices that shuffled identifying information every day.