We have conversations (see githup) as basic and inside this we need some modifications.
We run our own xmpp server and separates sip server with csipsimple. By api the registration in conversation is there one sip and xmpp users. Under the conversations user can call other user in his contact list. For this there is already some working examples.
Then we need some changes in conversations and some small cosmetics changes in csipsimple.
1) All apps are up and running
2) we need just modifications ( i will send details in chat when you are reliable freelancer)
3) you don't have to write complete new app.
4) only freelancers with experience with privacy chats and voice/voip
5) your bid is your bid, no place holder!!
6) after done, we want the sources!!