It is related to the signal strength of your mobile network operator. If the phone signal is poor, the sound might be delayed.
In this case, we advise you to change the room or to go near the window or outside (if you are in the building). The best mobile network coverage should be outside.
Also it is useful to check the status bar symbol which displays your cellular network’s signal strength before making a call. One bar means that the signal is weak, five bars mean that the signal is strong.