When you make a call, your mobile phone's signal gets 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 connected. If many calls are being transferred to a particular site or if there's a software glitch, you might lose connectivity.
Mobile black spots are another reason that you might lose connectivity. In rural areas, black spots or dead zones exist, because the distance between cell sites is quite large. In urban areas, black spots could be because the signal is blocked by buildings, or you are too far underground for the signal to reach your phone (for example, basements and parking garages).
Check network status