Nokia 5610 Xpress Music — RESTART

hello sir,
i bought Nokia 5610 on 8th jan 2009 ( IMEI NO.--------[protected] and product code[protected] and job sheet no[protected] / 090203 / 20 ). and since very first day it has problem .it becomes hang then display off and again after some time it becomes on, specially at the time of calling.
i submitted it at NOKIA CARE three times but the problem is still going on .but my cell is not changed and told me that nokia never change the phone.
now i want to know[protected]
1. why my phone can't be changed while it has the prolem since very first day?
2. who is responsible for the defect in phone?
3. why customer should suffer for that economic and mental harrassment.
4.who is responsible for wasting my time to see again and again nokia care to listen hard words and rude behave from people orking in nokia care.

PLZ REPLY ME .