Not getting mobile service even in warranty period

my hand set nokia lumia 720 is not getting service even in warranty period..
IMEI NO -357297054103396 ,job sheet no:410465550/140807/017
this is the third time my mobile is getting some problem..
after taking brand new mobile set through flickart within one month the mobile display starts flickering and i approached nokia care in trivandrum.They had send that mobile to Delhi for service and after about 2 months i got that mobile back..then again ith gots complaint with the micro phone.then again i approached a nokia care at Attingal,kerala.then those guys opened handset and they told me that they had to send that mobile again to Delhi.So i brought my mobile back and started using my head set instead of microphone..and now again my mobile got completely dead..This time i again approached a nokia care at Kollam..because I am fed up with both the previous nokia cares..My sets warranty period is till september..Now they are telling me that i wont get warranty for further service even in within the warranty period.After taking my hand set i loosed around so much time because of this hand set and i really want a relief from this.Now my hand set is with Kollam nokia care service center(NCC CODE-410465550 E MAIL [email protected]) and they are asking me to pay rs.600 for checking the handsets problem and further want to be paid if there is any damage..What i shall do now>??