I am developing an Android application, in which I want to make a calls using internet using SIP
in android. So I need to maintain my own SIP sever for my app users, how can I create my own SIP server?
I would not advise creating your our sip server as it would take a large number of man years of development and there are a lot of pitfalls.
There are some open source implementations that you could install and setup yourself. Like FreeSwitch or Asterisk. Both are large and complete to setup as there is a lot of domain knowledge required to understand how to set them up correctly.
There are also free server that you could try out as well like Sip2Sip.
Then there is the job of creating a sip client on Android. Again it's not that simple either. I would look at using a open source library here as well, like pjsip. This gives you the advantage of being able to look at examples of full sip clients already developed for Android like csipsimple. pjsip also has the advantage of being cross-platform, so you could reuse it in IOS for example.
Good luck.