I need to integrate VoIP service in my Android application.
Is there any free (best open source) library ?
I need that the library at least allows voice call over internet and not necessarily internet to phone (like 2 Skype account).
From android version 2.3 SIP API is added in Android. you can use either inbuilt SipStack library or third party open source sip stacks.
Refer Android Developer's Guide
There are different open source sip stack libraries as well as projects are available on internet. You can download the source code of that projects.
Here is the List of some popular open source sip stack libraries which allows to voice call over internet.
1.Pjsip
2.Mjsip
3.doubango
4.belle-sip
There are different open source projects which have used these libraries in their projects.like Sipdroid uses MjSip, Csipsimple uses PjSip, imsdroid uses doubango and Linphone uses belle-sip.
You may also like to refer this Android SIP stack - what to use?