Anyone know what tech stack/language the SDK is in? Java? C++?
They need to be able to run from both .NET (Windows 7 devices), *nix (embedded systems), Java (Android, BlackBerry), and C/C++ (iPhone) environments.
Most likely it'll be some sort of SOAP/REST API for authentication and management combined with a SIP or SIP-like standardized VoIP protocol.
Most likely you will have a relatively simple C api (not C++, as the C++ isn't likely to be portable across compilers).