Build Invite-Only Android Voice Calling App

I need an Android app built from scratch - a voice calling app, nothing else. No text chat, no voice messages, no video. People call each other one-on-one, or jump into a group room where everyone talks together, up to about fifty people.

Two things make it different from a normal app. It's not going on the Play Store - people install it from a link I send them. And it's invite only, so nobody gets in without a code I hand out.

Inside the app, people sign up with that code, add each other as contacts, see who's online, and call. Mute, speakerphone, block someone, and a list of past calls. The part that matters most: when a call comes in, the phone has to actually ring and show it on the lock screen, like a real phone call, even if the app was closed. If that doesn't work, nothing else matters.

I also need a small web panel for myself - to create invite codes, see who signed up, turn someone's access off, and look at the call list.

So that's the app, the server behind it, and the panel. I need it running on a real server, tested on real phones - Samsung, Xiaomi, Oppo especially, because those kill apps in the background and that's where calls get lost — and the code handed over to me at the end.

What I'm looking for is someone who's built a voice or video calling app before. That's the one thing I won't compromise on. Tell me which one you worked on and what the hardest part was, how long you think this takes, and what you'd charge.