1. Go to getonsip.com
2. Create your free account.
3. Get details like server, proxy, username and password.
4. Setup SIP account on your Xperia by going to call settings> internet calls>Account. Add sip account and enter details there. Save the account.
5. Enable data and enjoy free internet calls.
6. Now save contacts in your phonebook as internet calls. You are done, enjoy.
Step By Step Tutorial With Pictures Is Given Below.