Audiovox PPC 4100 / SP230

I was recently gifted a new PPC 4100 by a friend. I am an Airtel subscriber. I am able to see my contacts list on SIM CARD after inserting it in the PPC 410o. But still the phone is not working and keep on searching for Network. ( The SIM is perfect and works well with other instruments). Is it something to do with "Unlocking the phone". The IME no. is 351961000129823. Thanks in advance for providing any information.