I'm not referring merely to landline phones, but any predominantly voice-based communications system in which any user anywhere can presume to reach any other person without some additional requirement.
I've asked this question a few times --- asking when people think PSTD might die, in 2, 4, 8, 16, or 32+ years. Results are of course nonscientific, though interesting, and have varied widely. (My pessimism isn't especially widely shared.)
I did discover a while back though that telephone engineers share the same concern and for largely the same reasons:
[S]ince mid-2015, a consortium of engineers from phone carriers and others in the telecom industry have worked on a way to [stop call-spoofing], worried that spam phone calls could eventually endanger the whole system. “We’re getting to the point where nobody trusts the phone network,” says Jim McEachern, principal technologist at the Alliance for Telecommunications Industry Solutions (ATIS.) “When they stop trusting the phone network, they stop using it.”
https://nymag.com/intelligencer/2018/05/how-to-stop-spam-rob...
When the telephone was first introduced, and as it expanded through the 1950s and 60s, it was, as Facebook in the aughts, an aspirational instrument. Truman beat Dewey, but pollsters, relying on wealthy- and Republican-skewed polling, got the race wrong. Service was expensive, long distance was insanely expensive. There was little automation, and if someone was calling, odds were quite good that you really wanted to talk to them.
What killed the phone is the same thing that killed email, Usenet, and is killing the Web: it got too cheap. Good for those with legitmate business, but unfortunately, a far larger improvement to those with illegitimate business. Penny-ante schemes become viable when launched at scale and from low-overhead locations.
A couple of instances of the poll (and discussions), here:
https://mastodon.cloud/@dredmorbius/102357651020681668
https://toot.cat/@dredmorbius/106869063626188801
For anyone keeping tabs on my accuracy, I'd suggested on 17 July 2019 that death of PSTN telephony was <5 years away. That's looking somewhat overambitious from here.