If some one disturbing you from unknown no, you can trace the location of their number. It may use for annoying prank calls, blackmails, unknown number in a missed call list or similar.
Even though it is not possible to trace the number back to the caller, it is possible to trace it to the location of the caller and also find the network operator.
There are two options here for you trace location and service provider of mobile number:
In wikipedia there is list of mobile number starting 4 digits and corresponding details.
In number plan site you need to analyse by entering the mobile number which you want to trace.
Number plan is best option to trace the mobile number