What is the ruling on praying the Sunnah of Fajr immediately after the the Adhan, then the obligatory prayer?

The time for the two Rak'ahs of Fajr (Sunnah) begins with the entry of the time for Fajr prayer. It is permissible to pray them when one is certain or has a strong presumption that the time has begun. Usually, muezzins do not call the Adhan until after the time has entered. The scholars have stated the permissibility of following a righteous muezzin who is knowledgeable of the times, because it is apparent that he would not call the Adhan except after the time has entered, and "the muezzin is entrusted." Therefore, praying the Sunnah of Fajr and its obligatory prayer immediately after hearing the Adhan is valid.