Back when this was all being built, people relied on the 7-layer model. Today it's been reduced considerably, probably due to laziness more than anything else, though with it's inevitable costs in efficiency and effectiveness.

POTS is a physical-layer mechanism. IP is in the upper layers albeit at the lower end of those layers. Whether you receive your phone service via POTS or some all-digital scheme is not relevant to the fact IP is the distribution mechanism.

My own interest in these matters waned once the money stopped flowing, at least to me. That was more or less concurrent with the wide acceptance of the current, flawed cellular comm's system, though that wasn't the cause. I do regret that the popular system was adopted because its reliability is only on the order of 1% that of the "good-old-circuit-switched-network" with which I was reared. Having used that, in all its incarnations since WW-II, I recall no interrupted calls or even missed syllables, aside from when I dropped the phone. That system had extremely low bit-error rates, and worked very well. The cellular system drops more bits per second than the old wired system dropped in a year. Yes, part of that is due to more bits ...

Let's not hijack Ralph's "Magic Jack" thread ... this discussion could go on but should do so in a different thread.

RE