Both fasting and the time of Fajr prayer begin with Imsak (the time when the day's fast begins). So, it does not make any difference whether the adhan (call to prayer) is announced earlier or later. In the Hanafi school of jurisprudence, since it is more virtuous to perform the Fajr (morning) prayer a little later, in the other months than Ramadan, they start performing the morning prayer after reciting the adhan after the imsak time. However, in the month of Ramadan, it is recited earlier to announce that the time for fasting has started.

If the adhan is recited at the same time with imsak, one must quit eating and drinking as soon as s/he hears the adhan. Otherwise, the fasting may be invalidated. The best way is to quit eating and drinking before the adhan.

The one who eats or drinks while the adhan is being recited by thinking s/he could do so must fast one day as qada after Ramadan.