I built an app that uses the accelerometer embedded in smartphones. I received a letter advising me that I am infringing on patents of the systems in the smartphone that monitor and track movement. Do those systems not belong to the smartphone that the user purchased? Or do I need to pay the patent holder of the system?
I'm afraid there is no way for us to advise you. You don't say who is telling you of the infringement and what the infringement is. It could be the way you are using the accelerometer that matters rather than that you are using the accelerometer. It could be a scam. We have no way to know and in either case, no one is going to give you a legal opinion without knowing a lot more. At the very least, the person claiming infringement should provide the patent number of the patent they claim is being infringed.
My recommendation is to consult with an intellectual property attorney.