We can&#x27;t have sidetone nor full duplex in wide area cellular telephony because end-to-end latencies become so long the far-end echos go beyond asthetically disconcerting.<p>Try this just for fun: Call someone right next to you via your cell phones and talk to each other.  One finds one can speak several words, maybe even a short but complete sentence before the other person starts hearing any of it.  It can become a laughably long latency when experienced so directly.<p>But this cumulative latency ( be it from queuing delays, voice packet grouping for burst transmission, buffering delays along the many packet switches from here to there, you name it ) comprise engineering trade-offs necessary for optimal radio-level multiple access, efficient packetizing, and economically efficient (packet) transmission.<p>In other words, I&#x27;d postulate we can&#x27;t (today) afford full duplex, sidetone-included, wideband (5KHz+) sub-20-millisecond latency in cellular telephony.  Cellular is a different thing, but surely it&#x27;s its own kind of magic.<p>But consider modern office telephony over Ethernet:  I don&#x27;t know the internals of it, but aesthetically, I find it significantly better than the analog telephony I remember from 40 years ago: The sidetone&#x27;s there, it&#x27;s full-duplex, speakerphone functionality is absolutely superb, and subjectively I find it sounds good.<p>I&#x27;d guesstimate a three orders of magnitude latency ratio between LAN-based telephony vs cellular telephony seems to me key to the very different engineering possibilities and compromises necessary in these two very different domains.