Should the app's call to prayer be taken into consideration, or that of the muezzin who is 20 minutes late, for the Sunnah of Fajr prayer and for abstaining from food and drink (Imsak) during fasting?

It is permissible to rely on reputable calendars, such as the Umm Al-Qura calendar, which is considered one of the most accurate calendars, even if some scholars believe it is early in the Fajr Adhan (definition). The decisive factor is the entry of the time, not the action of the muezzin. If certainty or strong presumption of the entry of Fajr time is achieved before the Adhan, then one must abstain [from eating and drinking]. If that is not achieved, then the reliable factor is the Adhan of the trustworthy muezzin.