The end game is everyone gets the shits, and there is a noticeable drop in the usage of the old POTS (plain old telephone system) network. People use Apple Facetime, Google Duo or whatever instead. Then the telco's start to notice they are loosing customers.
At that point one of two things happen. One is that telco's fix their networks. The second thing is they decide it isn't worth the effort, and let the traditional phone system die. Given phone calls are effectively free so there is stuff all revenue in them, I bet it's the latter.
If that happens it will be painful. Like it is with messaging now, but even more so. Messaging now is either SMS with it's limitations (like you can't use it from a computer), or a choice of a zillion walled gardens - Apple, Hangouts, Slack, Signal, Viber, Telegram, WhatsApp, Facebook, ... most of which I don't have installed so I can't communicate with someone using them. The voice equivalents are Facetime, Duo, Viber, Signal - many of the same things in fact. The result will worse than messaging - the ability to communicate universally with anyone dies, but with no SMS fallback.
But that's not the end point. Universal communication is just too useful to be dispensed with - as the explosion of internet and the postal system before that have shown. So something will replace it, and once again we will all be able communicate with anyone we please.
However, the replacement has to solve the parasite problem. Once the cost of sending a message drops below a certain point every universal system we've had so far has been overrun with parasites, aka spammers. The postal system has junk mail, email has it's spam, now the phone system, and of course SMS.
A solution may be to allow the recipient to charge the sender any amount they like for successful delivery of a message. Most people would allow friends to send for free, messages from unknown recipients to cost something, messages from spammers cost more.
That could happen with the existing phone system of course, but I'd lay log odds the incumbents have too much in common with the dinosaurs for it to even cross their minds. Sadly that means we are in for a very painful transition period. In fact they are already losing customers as people stop using land lines in droves, so I'd say the writing is on the wall.