Pabitra, you need to remember that we can not see your devices and so we have no idea if you have only one downloaded app or hundreds.
Usually, the apps you download from the AppStore are safe, but on occasion a bad one slips by.
At this point, you could disable the third party ones, and only allow a few cellular access. By process of elimination, you can try to find the one that is the problem.
IF and I do mean if, one of them is a problem. It might just be an issue at your provider's end.
Several years back, I gave a cell phone and was paying for top off cards for a friend. He was working for me and I needed to be able to reach him. I also set it up so he could call our distant mutual friend. But he seemed to be burning through the cards like crazy. Mike, being technophobic, I checked his online account, and saw that a lot of toll or added value calls were being made, but from being made first from the US west coast and then minutes later from the east coast. So I called the provider, and asked if phone cloning was still possible. They said no. Then I asked them how the account could make calls from both coasts within minutes of each other. Final answer was that it supposedly was an error in their billing computer, to the tune of nearly $300 in just a couple of months.
While it is most likely a rogue app on your phone, that is not the only possible answer. Especially since it is going to a VAS. Do a web search and see if there are other complaints about charges for that particular VAS.
