It depends on the phone model and carrier, but generally speaking, using a foreign sim card while in India will void the warranty of your Indian phone. To be safe, you can call customer service and ask to be transferred to their roaming department. They will be able to help you in resolving any issues you may have with using your Indian phone with a foreign sim card. Even though using a foreign sim card may void the warranty of your Indian phone, you can still use it as long as you have activated it first.