For most people, its a permanent fixture on their person: people these days leave their home more than they leave their phone.
> Echo/Home are explicitly audio surveillance devices, though ostensibly benevolent in their design.
Echo/Home are no more explicitly audio surveillance devices than cell phones are.
> I can't say the same for Echo/Home, and it somehow feels qualitatively different in that phones aren't designed to be always-on microphones constantly processing audio to send it off to a server somewhere else for analysis.
Modern phones (considering the hardware and software stack together) are, in fact, designed to do just that, both Siri and Google Now voice recognition works (though it is, IIRC, disabled-by-default in the latter case) from the lock screen. Its just as always-on as Echo/Home.