The limit set by Allah is the true dawn, not the sound of a particular muʾadhdhin: "eat and drink until the white thread of dawn becomes distinct to you from the black thread." If the adhan is being given at the correct time for dawn, you stop at once and put down what is in your hand. It is agreed upon that Bilāl used to call the adhan while it was still night and the Companions kept eating until Ibn Umm Maktūm called, which shows that an early call does not end suhūr; likewise, if you are genuinely unsure whether dawn has entered, the default is that night continues and your fast is valid. Some scholars, on the basis of the report "When one of you hears the call while the vessel is in his hand, let him not put it down until he finishes his need from it," allow completing what is already in hand — though other scholars weakened that report or understood it of the call given before dawn — so taking the precaution of stopping is safer and clear of doubt.
Q&A · Fasting
If I am still eating when the Fajr adhan is called, must I stop immediately?
Evidence — the daleel
Quran 2:187Sahih al-Bukhari and Sahih Muslim (agreed upon), hadith of Ibn ʿUmar: 'Bilal calls the adhan at night, so eat and drink until Ibn Umm Maktum calls the adhan'Sunan Abi Dawud 2350
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.