According to the Federal Communications Commission, the topography of an area is one of the most significant factors contributing to cellphone dead zones. It can occur in rural areas or in big cities. Dropped calls can occur when you're on your phone and walk into a building, or drive into a tunnel, both of which can cause a physical obstruction blocking the communication signal between the phone and the nearest tower. This can also happen in areas where the terrain is mountainous. In addition, dropped calls can happen when a you're traveling from one cell site to the next and the call isn't handed off properly between the cell sites. Does this happen only inside? If so, there may be something in your home obstructing the signal. Some construction materials, such as rebar, may do it. If it does it outside, it may just be location. Mountains or hills? Distance to nearest tower? It could be many reasons. You need to try and identify the reason.
Sent from from my ancient but trustworthy iPhone 5