Are you trying to learn how this works or just need a landline?

If you just need a landline, the ISP will give it to you. It’s voip anyway most of the way. Just plug regular phone into router done. Maybe needs to be a proper voip phone in which case your ISP will give you connection details. Usually that connects to a voip server running in your router.

I worked at a voip provider so I want to mention: this is not a “set it and forget it”. This is something requiring regular maintenance with pretty strict regulations from the providers side. Like email, most consumer ISPs are very wary about people self hosting it.

If you are “pretty new to networking” this is not the project to start with, because it requires solid networking fundamentals.

If you really want to go anyway, well… You need something like kamailio. You set that up, get a voip phone that you have to program to register itself. Plan a couple weekends for this because those docs are not for the faint hearted and you probably have to read the source to understand it.

Forward SIP and RTP from the internet to your server (do you have a static ip?). Then you need to find a provider that gives you a number and is willing to have residential connect to it. Then you hope your ISP is not blocking those ports.

Then you should set up some kind of monitoring system because those providers are pretty flaky and will tell you stuff is working when, in fact, it’s not.

The dev seems surprised by what is pretty common knowledge:

• You get only roughly 50% of gross revenue paid out after distributor cut (30%), vat (10%) and returns (10%).
• Games make the vast majority of profits in their first month, only ticking back up slightly when releasing DLCs.

So there are only three things to do really:

• be happy the game did well, do 1-3 months of patching, and go on to the next game (and do your best to ignore angry gamers yelling that “the game is abandoned”)
• the paradox route: keep releasing regular DLC to keep cash flow up while also releasing regular free updates (and do your best to ignore angry gamers yelling that you are a greedy evil corporation)
• introduce micro transactions, subscription models or any other way to keep making money after release (and do your best to ignore angry gamers yelling that you are greedy)

Instead, for some reason the dev seemed to think they can keep up developing the game indefinitely and that somehow, it would keep making money? There are like 10 games in history that made enough money to allow development for years: Minecraft, Terraria, Witcher 3, Stardew Valley, etc. And I don’t think they are even cash flow positive on their own.