Hardly. You're going from analog to digital and vice-versa. You probably need specialized appliances. For every country in the world. And it's "solved" but only in proprietary contexts; I don't think there's a standard. Then you need to operate it - you need SREs, bug fixes, keeping up with downstream changes etc.

Adding PSTN to Discord is absolutely a Discord-sized problem.

>Software is cheap and easy to produce these days

Yeah todo list apps