Microphones are working properly, but can’t use it for video call

It seems your upper microphone is not working, how did you determine all three are working, did you try recording and playing a video with your front camera ? If you can record videos it should be working with net applications too. It might be just a software glitch, try first with a hard reset (power+home button pressed until reboot) and if that doesn’t work you may try a DFU restore if you can use a computer. Don’t forget to backup first as restore will wipe all data.