Greetings lubylukose and welcome to Voipuser
The best solution will always be to use a simple VOIP softphone or an ATA in both locations - calls between the two will always be free. This does however assume that you have access to broadband in both locations and that your local ISP's do not block VOIP traffic.
Calls to PSTN are never free - they are paid for by someone, usually as a part of a marketing programme, company promotion for a limited period of time or as part of a package where some elements are paid for. There is also no such thing as 'unlimited calling'. All services are subject to some kind of 'fair use' policy
That being said there are good deals to Oz from the Betamax/Fianrea guys, see list here ...
http://backsla.sh/betamax
If you have to resort to PSTN then you may also get Jajah (.com) or Nonoh (.net) to work free with both numbers registered.