Voice chat/voip implementation windows phone 8

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Windows-Phone-8

Hello,
I have a project where I have to make a community for people where they can offer some services for example.
But one of the main parts of the project is that each user can call a specific user over ip or in a voice chat way.
The service should be free;therefore, I am looking for other suggestions that using SIP protocol.

Is there even a dummy way to implement this in wp8 because this feature is the main feature in my app.

Regards