Mobile Number Portability has been launched in India and it’ll give a lot of mobile subscribers the freedom to change their mobile operators while retaining their number. Most people have given reasons like “bad service”, “expensive” etc. for switching operators but one more important factor to consider is whether your new operator has a 3G license in your circle.
For example, in Delhi/NCR region, only Vodafone, Airtel and Reliance have 3G licenses. So if you move to an operator like Docomo, Idea Cellular etc. in Delhi, you will not be able to enable 3G services on your handset. So your high end iPhones and Androids will be stuck to the plain old GPRS
The complete list of which operators own 3G license in your circle is given here.