Is it not permissible to pray Fajr except after the second call to prayer?

The meaning of the second call to prayer (adhan) for Fajr prayer is the adhan that occurs at the rising of true dawn (Al-Fajr As-Sadiq), and it is the one after which the Fajr prayer is performed. It is not permissible to pray Fajr before this adhan because it occurs before the time for prayer begins. This is indicated by the Prophet's (peace and blessings be upon him) saying: "Indeed, Bilal calls the adhan during the night, so eat and drink until you hear the adhan of Ibn Umm Maktoum." However, if the prayer is performed after dawn but before the adhan, it is valid.