not getting back my phone from service center

Location/place: Hyderabad,A.P.

Name of company/service: sony ericsson, mobile phone xpria SK17i

I bought sony xperia SK17i on 2nd March’12, but the touch screen stopped working so I went to service center on 21st Aug’12. I have not got my phone back. I called several times to the service center and customer center, but it did not work.
please help me get my phone back.