When you get a call from someone, your phone screen displays the caller's phone number but wouldn't it be useful if the screen could also show some additional information about the caller – like where they are calling from?
If the person has made a call from a landline number, you can easily determine the caller's city based on the STD Code of the incoming call. For calls originating from mobile phones, the caller's state (or telecom circle) and their mobile operator can be quickly identified from the first four digits of the mobile number as per this chart.
To give you an example, if the incoming phone number is 9210xxxxxx, it's a TATA Indicom number registered in the Delhi circle which includes Faridabad, Ghaziabad, Gurgaon and Noida.
You can perform these database lookups manually or, if you have an Android phone, get Shaplus. This is a free app that will display, while the phone is ringing, the caller's location based on their number (see screenshot on the right). It works for both incoming and outgoing calls.
Shaplus also adds an icon on your phone's home screen to help you perform lookups offline. You just have to key in the first five digits of the number to know the city or the telecom circle and the cellular operator for that number.
This will however not work for mobile numbers that have recently undergone operator change under the Mobile Number Portability (MNP) scheme.