Voip sip client for Android - cross-platform tools for mobile development (Xamarin, C#)

Hi,

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 (voip-sip-sdk.com/p_456-how-to-build-an-android-voip-client-voip.html) 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 (xamarin.com) 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?

Thx, bye