VoIP SIP SDK provides a powerful and highly customizable solution
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VoIP SIP SDK provides a powerful and highly customizable solution to quickly add SIP based dial and receive phone calls features in your software applications.
It accelerates the development of SIP compliant soft phone with a fully-customizable user interface and brand name. The ABTO LLC VoIP SIP SDK contains a high performance VoIP conferencing client capable of delivering crystal clear sound even for both low and high-bandwidth users and SIP compatible devices (hardware and software).
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Since I cannot really integrate it into my system, I finally decided to use Ozeki VoIP SIP SDK. I implement the MediaGateway example from their webpage - it ensures communication between two Silverlight clients via a SIP PBX.