Sony Ericsson - Mobile — W810I - Hanging Problem

Dear Sir/Madam

I had purchased sony ericsson W810I mobile phone in October 2007 and its in warranty. And i facing the problem that it got hanged and battery is not geeting charged. It has been more than one month that my problem has not been resolved. i deposited the mobile to authorized service centre 3 times and still it is with them and now they say that it will another 15-20 days as the will change its port. i even contacted the customer care and it has been found that my case no is closed without even having a call from them. This is pathetic service which I am getting from such a big company. I am facing a lot of inconvenience because I m not having any other mobile. They are still not making any progress.
I have alll the details of the complaint and the complaint number also. So, could you please tell me how to move forward now. I want to lodge a complaint now.