IMEI no. can be used by the network provider to impose a sort of “ban†on the stolen cell phone,i.e, debarring it from accessing any network.
With this IMEI no. your phone can be traced. As u have filed an Fir. If your brother's lucky it will be trace within a month. I lost my Nokia N-gage way back in 2006 it was traced within a month in Jalandhar. If the guy who has your cell changes the IMEI no. then your brother can say bye-bye to the phone. Now a days people mostly go to the local mobile repair shops to change the IMEI. I dont know how but maybe using something from the same model of any broken mobile that is present in their shop.Not sure.