Apple VOIP Push notification server

Question

So, i'm developing a IOS VOIP app, and find out that i need to use a Push notification server to make my app work on background to receive incoming calls and etc.

I searched and dont find anything about how can i set-up, develop, use, find about, a push notification server/gateway

I found some server who are paid and dont help me on anything, because i can't pay it right now.

I'm sorry to write a question entirely of text, with no code, but there's no code to use right now.

Yes i read the documentation, and articles, and etc, but they just talk about "Setup your server and send a push to apns", nothing useful about what i should use or how can i set-up a server.

Here's what i already found about it:

Mizu VOIP push gateway

How to create a native iOS app that can receive VoIP push notifications

Session Talk push gateway

Mizu VOIP Push notifications

Push notifications docs

In case doesn't exist a non-paid alternative to a push notification server/gateway, is there a workaround to this problem with the newest IOS changes? (I found something like a workaround said by the Zoiper team here)

Thanks in advance!

[EDIT]

The solution was using the Flexisip proxy server.