What is the prescribed time to start fasting from? I have heard that when azan is listened, sahri time is not ended yet. Thanks
All perfect praise be to Allah
The ASOOL is, that sehri time ends when it does(when dawn breaks), it is not restricted with azaan...
So, you should follow a credible source of suhoor timing and end sahr before that end time.
Now at some or most of the places, fajr azaans are called exactly on the time of end of sahr, in which case if you are sure they make sure they call azaan at proper time, than you can follow the azaan, the thing is you have to make sure you are following the sahr timming.
And during azaan you must not eat, but as of some ahadees, if you are drinking water lets say or you are eating something and a little is remaining and you are still hungry, than you should finish that thing. Prophet ﷺ said:
It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “If one of you hears the call (to prayer) and the vessel is still in his hand, let him not put it down until he finished with it.”
Narrated by Ahmad, 10251; Abu Dawood. 2350;
Also have a look at: https://islam.stackexchange.com/a/1776/15642