First of all never use the term 'Unlocked.' In Banglore/India 'CELL PHONE LOCKED' means your cellphone's keypad is locked. So if you say "my phone is locked" very few people will actually understand you and fix you're problem. My suggestion is, if you're living in the USA buy 'PAY As You Go' phone from AT&T. You can buy them from Bestbuy. Or if you already have a GSM-phone you can use it as well, it will work in India. Only thing you've to buy a SIM Card from the local service/connection provider, once you're in Bangalore/India. You'll need ID proof to get a new connection. A photocopy of your passport will work.
In India two connection providers/carriers are quite popular and reliable too: Vodafone and Airtel, I have used both.
I like Airtel due to its connectivity. If you already have a 'Pay As you Go' or Go phone/GSM phone just replace the SIM card with a new one, once you're there. Also, there are plenty of neighborhood shops from you can buy SIM cards. Such shops will have a sign board/bill board right out side which says 'STS/ISD.' You can make ISD or international calls from these shops as well. But BE VERY CAREFUL. Often times they fleece people. Also, SIM Cards are not calling cards.
From prepaid SIM card you make both international and local calls. You can call USA too. Incomings are free in India from any carrier/providers.
When you've used up all your outgoing minutes, you can recharge from the same shops from where you've bought your SIM card/handset. The executives in these shops can also help you replace your old SIM card with a new one.
You can buy SIM cards from the following places in Bangalore. For Airtel connection, visit any of these authorized shops. There are other shops, but following ones are reliable. Go in the morning, it will be easier to locate:
AIRCOM SOLUTIONS
37 2 Chicago Avenue Cunningham Road Bangalore 560052 560052Bangalore
TOUCH TONE MARKETING
25 7th Cross Road 2nd Main Indra Nagar 1st Stage Bangalore 560 038 560038Bangalore Nil 9 30am-8 00pm -
BRANDSHOP INDIRANAGAR LOBBY
Ground Floor Salarpuria Landmark 2002 100 feet Road HAL 2nd Stage Bangalore 560038 LM Near Coffee Day Citibank ATM 560038Bangalore
Other Aritel locations are listed here (works only on Internet Explorer): http://www.airtel.in/wps/wcm/connect/airtel.in/Airtel.In/Home/ForYou/Mob...
Following Vodafone outlet is reliable, located very near to FORUM MALL. I've visited them couple of times:
Vodafone Store
#113, Prestige Pinnacle, Koramangala Industrial Estate,
Opp. Raheja Arcade,
Koramangala, Bengaluru-560095
11 am - 8 pm
Sunday (Closed)

Refer to these sites
http://www.vodafone.in/existingusers/prepaid/pages/storesndealers_kar.as...
Select Vodafone Store
http://www.vodafone.in/existingusers/prepaid/goprepaid/pages/goprepaid_k...
If you leave Bangalore and go to some other states in India, like Mumbai you're roaming will be activated automatically for prepaid, but first check with the service provider by calling the customer service department, or inquire when you buy the phone.
In the airport you will find STD/ISD booths, similar to the neighborhood shops I've mentioned above. In the airport these shops are reliable. Just ask any airport personnel, they'll guide you. Do not ask locals. You credit card will work only it is international. Call up your credit company and find out whether it is activated for international use. Or check your credit card, usually for international credit card it'll be written 'INTERNATIONAL' on top of your card and it'll work in Bangalore. Do use it legitimately. Use only in big establishments.