Sony Ericsson - K810i — Phone started malfunctioning within one month of buying

Hello,
I bought a Brand new Sony ericsson K810i handset from mobile store Cochin worth rs 11900/-.Within 2 weeks it started malfunctioning i.e. the keys got hanged, shortcut keys popping up and i was not able to make a call. I went to mobile store but they adviced me to go to Authorized service centre which is in Kalabavan road, Cochin.I went there and as i dont know malayalam and as i was alone they told that it had liquid damage and made me sign one form.After that they made me pay 540 Rs/-. I was ready to pay the amount as i had no time and as desperate to make the phone ready.nearly after 2 weeks they returned the phone back claiming that they have repaired it.Within 2 days it again started to repeat the same problemsThen i went to mobile store and they told me that i have made a big mistake by paying the amount. They also told that if i get a DOA certificate from Service center i will get the phone replaced.I was really emberrassed by talking to the persons in Mobile store.I called Sony Ericsson Call center and raised a trouble ticket and they again adviced to go to service center and as i have no other way i again went there and after talking to RM of service center submitted the phone.He told they will not issue DOA and for issuing the same there should be only 5 min talk time.Within 2 days of submission the RM kept on calling me to take my mobile back and he told that he has repaired.I got the phone and after fewithin same day same issue repeated after i realized that the Service centre oficial was lying to me.When i called the Call center they told that i have to change the PCB and it nearly cost 70% of handset.I realized that i was totally cheated.When i talked to official they told that they cannot do any thing and i have to suffer with the problem.