I am going to build a Cordova plugin that can connect to a sip server for both android and iOS version.
The program requires the usage of Linphone SDK.
I tried to download Linphone SDK for both android and iPhone.
First for android, I only found .jar file.
How to use it for my project.
I don't know what .jar file you found but it's probably not linphone-sdk. It is distributed as an .aar file through Maven.
You can also build it yourself, following README instructions from linphone-sdk git repository.