Vaibhav_Vicky: did you leave your phone at home? Do you live alone? I've heard of this type of thing happening before. One person I know visited another country for a couple months. While there he took a trip for a week to another country. Not needing his phone, he left it in a drawer in the house he was staying in. When he returned to Canada he received a huge bill for roaming and long distance calling. His phone was password protected so he couldn't fingure out how this happened. He eventually found out that housekeeper or some other person with access to his room, had removed his SIM, placed it into a another phone and made lots and lots of calls. The SIM was put back before he returned and he was none the wiser until he got the bill.
If you don't live alone, it is something to consider.