A number of popular apps are available now to trace out details for unknown phone numbers. TrueCaller or Mobile Number Locator are example of such apps listed in top free apps chart of Android play store. Though these apps can reveal many details about the number, using them can be a major privacy concern.
If you are only interested in knowing the geographical location of the cellphone number and operator details, Wikipedia can help you to find out.
In India, mobile phone numbers can only start with 7, 8 or 9. And it must have to be ten digits long. So DoT has defined the numbering format as XXXX-NNNNNN, where XXXX is the Network operator and NNNNNN is the subscriber numbers.
The XXXX is fixed for every operator circle wise. Here, we are making use of that only. Open this wiki page and use the browser's shortcut key combination Ctrl+F to find out the first four digit (without any country code) of the mobile number.
For example, if the number you are looking for is 9582123456, search the wiki page for 9582. The number will get located in a big table over there. In this case you will find something like this:
This shows that the number is of Vodafone telecom operator and it's home location is Delhi. Wasn't that really easy to locate the number?
The page lists abbreviation for more than 20 major network operators and telecom circle codes.
In this page you can find every number for 9xxx series. However, 8xxx and 7xxx number series table are partially empty and not all new series numbers are available.