how to play voip when the UI leaves the foreground?

how to play voip when the UI leaves the foreground?

we know microsoft has shown a voip sample
here:http://code.msdn.microsoft.com/wpapps/ChatterBox-VoIP-sample-app-b1e63b8b
now I have developed a voip program but I can't play voip when the UI
leaves foreground.
The structure of my program differs from the sample.
The thread capturing and rendering the voice is placed in the windows
phone dll, which is called by winrt component.
My UI will call the method in dll through winrt component.
I have viewed the sample codes and found that it has a server to connect
the foreground process and background process,
I can't make sure if the server ensures the performance of playing the
voip when the UI leaves foreground.
Could someone analysize the sample code and explain to me the reason? or
Tell me how to play voip when the UI leaves the foreground?
thanks~