I live in a Muslim-dominated locality. Some of my friends who are Hindu also live in the same area. There are close to 4 to 5 mosques in the vicinity not too far from each other and they call for prayer at the same time (plus minus a few minutes). My friend asked me a question if Azaan from the mosque is a call for prayer to the people, then won't a call from a single mosque will do the job? Why does every mosque in the same local vicinity need to make the call for the prayer?
If there are more than one mosque in the same vicinity, calling azan from only one mosque is not an option as prayer will be conducted in each mosque, so the call has to be from all mosque. When azan is called at same time from multiple mosque, it can be very much annoying to people there. So one solution to this is to use loud speaker in only one mosque and rest of the mosque they can call Azan without loud speaker.