Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.105B.
Section : The Iqamah of One Who Is Not the Designated Caller.
If someone other than the appointed caller wants to make the azhan, he must obtain the latter’s permission. If the appointed or regular caller is late and they fear that they will miss the time of the azhan, another person may make the call.