Sehri Time According To Qur'An
Azan is supposed to be delivered when the time of the prayer comes. if time is there, you can offer prayer, no matter whether Azan is delivered or not. In our country, there is a practice that Azan of Fajr is delivered a little earlier before its appointed time during Ramzan to make people keep fast within the time because our people wrongfully associated the end time of Sehri with Azan. So, if Azan is delivered before its appointed time, you cannot offer fajr prayer, unless its actual time comes.