您可以开始研究这些... [ ^ ]

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Hello,

I just started programming with Windows Mobile using Visual Studio. I need to develop a softphone based on SIP (Session Initiation Protocol) which runs on Windows Mobile. After a week of research, I have not found an API or SDK that is well-documented and allows to do the thing. And, most are paid. Does someone already have an API for this? I would appreciate any help! Thank you!

You can start researching these...[^]