"Where is it instructed that one should not talk at the time of AAZAN?"
There is a statement we found that may be of some help.  It follows:
1) When the believers hear the Azan they should listen to it in silence and repeat each phrase of the Azan in silence immediately after the Muazzin has finished saying the phrase.
2) When the Muazzin says:- Hayya 'alas salah and Havya 'alal falah - the listener should say in reply:- Lahawla Wala quw-wata Illa Bil-lah.
3) When the Azan has been completed, the listener and the Muazzin recite Darud unto Prophet Muhammad(saw) followed by a Du'a.

The AAZAN appears to be prayers to "Allah".  The True Christian  believes that the only True GOD is Jesus and thus all prayers should be directed to Him.  I don't want to load you up with a lot of information at this time.  But, if you are interested to know more about the all loving and all knowing Jesus Christ, please write me and we can have a discussion.