Is it permissible for someone other than the regular muezzin to give the call to prayer (Adhan)?
Chapter on Adhan
Al-Mughni
Book of Prayer
Primary text
Another person is not permitted to give the Adhan before the regular muezzin, unless the regular muezzin is absent and there is a fear of missing the time for the prayer. In such a case, another person may give the call. This practice is supported by the narration concerning Ziyad ibn al-Harith al-Sudai, who called the prayer for the Prophet, peace be upon him, when Bilal was absent. Similarly, a man gave the call to prayer when Abu Mahdhurah was absent before him.
Supporting text
When the regular muezzin is present, no one else may precede him with the Adhan. This is based on the precedent set by the time of the Prophet, peace be upon him, whose designated muezzins were not preceded in giving the call.