Answer
(Fatwa: 903/903=M/1430)
If the imam was assigned to call azan along with imamah (leading prayer), he has more right to call the azan. If he is asked to lead the salah only and there is no regular muezzin appointed, then anyone who comes first can call the azan provided he wears Islamic complexion and can correctly pronounce the azan. There is no need for the imam to re-call the azan. But, if someone called the azan in advance of time, it will be repeated when the due time will occur.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband