In some areas there are two azans, one is for Tahajjud and another is for Fajr Namaz. You asked about Farz azan so as soon time has set in for Fajr and Moazzin said Allahu Akber, now you are not allowed to eat or drink. Not only that, if you had water in you mouth and you heard the azan and if you swallow the food or if you have water in your mouth and you drink it, your fast will become invalid and you have to pay kaffara also. So suppose if you are eating and food is in your mouth and you heard the azan now you have to spit it out other wise your will become invalid.