For a number of years, they would continue to eat until the end of the adhān. What is the ruling of this?
The adhān of fajr can be before or after the break of dawn. If it is after, then a person must stop eating and drinking at the first sound of the adhān. This is because the Prophet said, “Bilāl calls the adhan at night so eat and drink until you hear the adhān of Ibn Umm Maktūm as he does not call the adhān until the break of dawn.”
So if you know that this particular muʾadhin does not call the adhān before dawn then you should refrain from eating and drinking by just hearing the adhān. If however he calls the adhān based on calculated timings or the clock then the issue is more flexible.
Based on this we say to the questioner you do not have to repeat these previous fasts as you cannot ascertain that you ate after the break of dawn but in the future you should be more cautious and stop eating and drinking when you hear the adhān.
Majmūʾ Durūs wa Fatāwā al-Ḥaram al-Makkī, Muḥammad b. Ṣāliḥ al- ʿUthaymīn, Volume 3, Page 52