Android VoIP App written in C# using Xamarin.Android

We are looking to have a simple VoIP app created for Android. The app must be written in C# using the [login to view URL] framework. The app needs to have the following functionality: -register itself with a Sip server -place VoIP calls -must work on WiFi or cell data network (3G/4G) if no WiFi is available -must use standard VoIP to be able to communication with any type of VoIP device -screen to configure login credentials to Sip server -we require a copy of the code upon completion The app does not need to be fancy. It just needs to have a few buttons to place a call, put the call on hold, and end call. The app does not need to be submitted to any stores. It does not need any special colors or images. It can be called whatever you would like. This will be the start of a larger app once the VoIP functionality is working.