Ian Bogost at the Atlantic has written a really pleasant nostalgia article about phones that could actually hang up on people, in a really satisfyingly forceful way.
Unlike today's cellular network, the public switched telephone network was robust and centralized thanks to monopoly. Apart from flukes like my son depressing the hook switch, a disconnected landline call is almost unheard of. By contrast, it's not possible to hang up on someone via smartphone with deliberateness, because it's so much more likely that the network itself will disconnect of its own accord. Every call is tenuous, constantly at risk of failing as a result of system instability: spectrum auctions, tower optimizations, network traffic, and so forth. The infrastructure is too fragile to make hangups stand out as affairs of agency rather than of accident.
Today a true hangup -- one you really meant to perform out of anger or frustration or exhaustion -- is only temporary and one-sided even when it is successfully executed. Even during a heated exchange, your interlocutor will first assume something went wrong in the network, and you could easily pretend such a thing was true later if you wanted. Calls aren't ever really under our control anymore, they "drop" intransitively. The signal can be lost, the device's battery can deplete, the caller can accidentally bump the touch screen and end the call, the phone's operating system can crash.
And he provides the necessary caveat to make this kind of nostalgia article uncomplicatedly nice to read, by doing the work for you in pointing out the silly things about it: "Lamenting the demise of hangups offers little more than crass nostalgia for an admittedly weird, anonymous aggression. It's pointless wistfulness, too, since the phone call itself has become an endangered species."