It is essential to carry a cell phone with you when traveling to India. This lets you manage your time better and to stay in touch with your social and business associates in your home country and in India. In addition, services like directions, taxi, food, medical and police are easily accessible with your cell phone.
You can take your current US cell phone to India and just use it if it is enabled for International calling (contact your service provider to make sure). However it is sure to be expensive to use (a phone call between India and US may cost between $1.29 and $2.49 per minute).
There is an affordable alternative that can save you as much as 90% of the usage cost. You can use your US phone by replacing its SIM card serviced by a local operator. Your phone must be triband GSM and 'frequency unlocked' (contact your US service provider to find out). In case your US phone does not confirm to this, you can buy a handset and use a local SIM card with it.
There are numerous places to get a handset and a SIM card in Delhi. You will have to fill out a registration form, submit your picture and a copy of your passport and have a verifiable local address. It takes about 24 to 48 hours to activate your service after all requirements have been completed and approved.