Thanks for reply. I have download it some time ago. If 3CX have a sdk, maybe a commercial one, it will be much more easy to build custom implementations and sell the Server to a lot of more people.
In theory / practice you should be able to use any good SIP SDK.
its not trivial to create a SIP phone though. What functions are you looking for? Its more likely that we would develop a high level API that removes the SIP complexity for you.