I'm trying to implement voip calling for Android (KitKat / version 4.4) using .net and the sip protocol and I need an open-source C# development library to accomplish my project.

So far, I used this android voip client example, but it seems my trial version will expire soon. Yes, it is... bad news.

By the way, while digging on the Internet, I'm thinking about using closs-platform tools. I'm especially interested in Xamarin that allows development with c#.

Do you have any knowledge about using cross-platform tools for Android-based voip development? Can you recommend me a Xamarin-compliant open-source c# voip library?