When you make a call, your phone’s signal is picked up by the nearest cell site. Your phone call will stay active as long as you’re within range of this site.
If you’re on the move the phone’s signal jumps from one cell site to the next to keep the call going. If many calls are being transferred to a particular site or if there’s a software glitch, you might lose connectivity.
Dead spots are also why you might lose connectivity. In rural areas, dead spots exist because the distance between cell sites is quite large. In urban areas, dead spots could be because the signal is blocked by buildings or you are too far underground for the signal to reach your phone (e.g. basements and parking garages.)
Check network status