When does abstention from food and drink (Imsak) begin in Ramadan; is it half an hour before Fajr, or at the exact time of the Fajr Adhan?

The time for imsak (abstinence) is when the true dawn (Fajr As-Sadiq) appears and becomes clear, based on Allah Almighty's saying: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night] at Fajr," and based on the Prophet's (peace be upon him) saying: "Indeed, Bilal calls the Adhan at night, so eat and drink until Ibn Umm Maktum calls the Adhan." If there is no Adhan, then one must look towards the eastern horizon. Upon seeing the true dawn—which is the faint, elongated light in the horizon—one should begin imsak. If there are Muslims present, they should convene to resolve this matter. Relying on the internet or calculations is permissible if the source is trustworthy.