It is believed that notice is not mandatory. However if notice by self or legal notice is given it has its own advantage and adds weight and force.
Learned Mr. Shah has given valuable advice. Kindly follow it. It is believed TRAI approved NGO's attend to complaints related to telecom operator's deficiencies in service/unfair practices.....It is seen in some of the issues forwarded to such NGO's, that many of the NGO's themselves, recommended to lodge a complaint with consumer forum's.
Chairman, District Consumer Protection Council and TV channels [pahredar programme of cnnibn etc] may be of better assistance and help, to get the replacement and may not be able to get you the compensation.
If IMEI number has been changed you can ask NOKIA to issue certificate to this effect, and if is required by rules/law they may update it on the bill under original seal and signature along with a letter to this effect.
Majority of the companies on receipt of the detailed complaint and intervention by resourceful entities may succumb to replace, however do not compensate for the mental agony/harassment.
In your detailed complaint you may describe cost of each visit ( describe kilometers etc), phone calls, professional/personal hours spent, emails, letters (typing charges etc) etc and demand refund. If you have any bills keep them safe.
You seem to have a good case.