@anne not bad...
VoIP, even with solid encryption, is not very heavyweight. I wrote a program that did it fine in python on a circa-2003 PC. Open VoIP programs usually plan on a graphical UI but you could make do with buttons or use Sphinx for voice control (though that is heavier, but people do it on a Pi).
You could even stick in a cellular module for use in data mode, if the latency were manageable.
@anne hmmm.... so one would have to reimplement the protocol either in hardware or on a super low power CPU?
Hmm. You're going to want Datagram TLS and/or SRTP, and those are a bit heavy for a microcontroller. (And existing implementations might be proprietary.)
On the other hand, careful work on a microcontroller might add robustness to unpredictable lags. Real-time programming there has teeth.
Whoa I love the idea of pulling out a metal retro phone and saying "hey Tux what's the atomic mass of xenon?"
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