The chat server requires constant network communications with various devices that change over time, suffers from possible supply chain vulnerabilities it needs to defend against, talks to databases, mobile apps—suffice to say, there's a million interactions that may be failing in the future due to unforeseen changes in the outside world.
A phone line introduces but a fraction of the complexity on-site, and thus doesn't need as much caretaking. I mean, ...duh?
And even if you want to pull through with this analogy; phone service customers still have an ongoing subscription to cover for the eventual repairs and wiring replacement. Just think of the transition to fibre in recent years. You pay for the service on the phone line and the maintenance of your infrastructure. It's a bundle.
What about a phone service? What about cable? WAN? Recent "smart" insanity notwithstanding, those things are not updated on end-user side; every now and then, some hardware or wiring might get replaced, but the infrastructure part on the customer side is generally one-and-done, and customers pay only for the actual telecommunication service.