Your phone may not work in India with any of the local Sim cards unless it is Unlocked. In most of the parts of the World mobile phones are locked to the Telecom Carriers. You will not be able to insert another sim or use it with another carrier. So if you are getting an iPhone from USA make sure its Unlocked. A lot of people are not aware of this fact. Hence they end up buying a new phone which is a complete waste of money. The new phone will not work in the native country. You may end up losing around USD 80-90 on this.
Let’s take example of iphone6 :
The iphone 6 is available in 3 models in USA-
A1549 is the most popular and is offered by most of the carriers. But if you are thinking of using these models in India then you will face a lot of problem connecting to the internet. These models do not support 4G LTE frequencies, therefore with these models you will not be able to take advantage of high speed internet. Though 3G is not that bad but for live streaming, booking tickets or hotel rooms and for sharing pictures 4G is the best. The Verizon version of iphone 6 also has CDMA and very few carriers in India support this.
Keep in mind that you need an unlocked gsm phone that gets frequencies used in India
2G capabilities GSM 900, GSM 1800
3G capabilities UMTS 900, UMTS 2100
4G capabilities LTE 850 (5), LTE 1800 (3), LTE 2300 (40)
These are not typically used in USA. So, foreign travellers generally end up buying a new phone. But, the phone you will purchase will not work at home when you return.
Wikipedia has a great article on phone bands so you can learn what you need and you can also look up phone band frequencies elsewhere that apply to your particular phone. Taking a few minutes to do this before you go will save a bundle of money over trying to buy some international coverage from your home cell carrier. Not only that, but having data on your phone in India is a real help in not having to depend on wifi in India, which is spotty at best. Sometimes wifi does not work consistently even in 5 star hotels.
You can get Trabug. It is a smartphone specially designed for foreign travellers and also comes with a 4G sim card. The difference between getting a new phone and Trabug is that you can get Trabug on rental basis. Delivery and pickup of Trabug phone is free of cost plus you can avail unlimited data pack. Get rid of the low speed of wifi in your hotel and enjoy your stay in India.