I am going to build pjsip on window7,and I almost be ready to compile the project,but something confused me is one of the steps show in this page:link
It is said about SDL2,describe is here: SDL sources comes with VS project settings, under VisualC sub-directory
So,what should I do indict by the description?
Another question,should I build SDL2 from source?
You should build SDL only if you want to compile PJSIP with video enabled. It is used for rendering only. If you want you can build it from source.
Another option for rendering is DirectShow but it's broken for ages. I developed a renderer driver for PJSIP to render to a bitmap and than I render this bitmap in C#. It's easier and works fast.
Also for video you will need ffmpeg at least for codecs. My opinion: start without video. It's not easy to make it work and might be you won't need it at all.