If you have IMEI no of your stolen phone then report it to your local operator to block the device. This makes your network provider to put your phone’s IMEI no in a shared database called as an Equipment Identity Register (EIR) which reduces the threats of theft of handsets by enabling individual operators to prevent the use of stolen handsets in their own network.
Now to make the stolen phone inaccessible in other networks (service providers) also, your IMEI no is transferred to Central Equipment Identity Register (CEIR) which is a database of the IMEI numbers of blacklisted handsets. Once the device number is listed on CEIR, it wouldn’t work with any of the network provider.