I think many of you may have come across this issue. Generally when we hear the Azan , we do the following:
Repeat what the Mu'aththin says, except for when he says:
"حَيَّ عَلَى الصَّلاَةِ وَحَيَّ عَلَى الْفَلَاحِ"
Hayya 'alas-Salaah (hasten to the prayer) and Hayya 'alal-Falaah (hasten to salvation). Here you should say:
"لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللهِ"
Laa hawla wa laa quwwata 'illaa billaah.(There is no might and no power except by Allah.)
But these days, if you happen to live in a muslim populated area, you will find a lot of mosques near by. I start answering the first Azan, I hear. Very soon other mosques also starts their Azan and it becomes very difficult to hold on to the first Azan. The voices of the Azan-s are almost similar. I am not sure what to do in this situation.