What are the rulings concerning the Adhan? And is it permissible to respond to every Muezzin who observes these rulings?

The Adhan is an announcement of the prayer time having begun, using specific phrases. It is a confirmed Sunnah for men, and some say it is a communal obligation (Fard Kifayah) for the five daily prayers and Jumu'ah (Friday prayer).

For its validity, the following conditions apply: the time for prayer must have begun (except for Fajr), it must be recited in Arabic, voiced loudly, performed in the correct sequence (tartib), and continuously (muwalat). The muezzin must be a Muslim, sane, discerning, male, and have the intention (niyyah).

Purity (tahara), facing the Qiblah, or standing are not conditions for its validity, though they are recommended.

The person who performs the Iqamah (call to commence prayer) usually performs the Adhan. For missed prayers (fawa'it), the Adhan is made for the first one, and an Iqamah is made for each subsequent prayer.

It is recommended for the listener to repeat the Adhan, just as they would the Iqamah. When the muezzin says "Hayya ‘alas-salah" (Come to prayer) and "Hayya ‘alal-falah" (Come to success), the listener should say: "La hawla wa la quwwata illa billah" (There is no might or power except with Allah). When the muezzin says "Qad qamat-is-salat" (The prayer has certainly begun), the listener should say: "Aqamaha Allahu wa adamaha" (May Allah establish it and make it perpetual). When the muezzin says the Tasweeb (for Fajr prayer: "As-salatu khairun min an-nawm" - Prayer is better than sleep), the listener should say: "Sadaqta wa bararta" (You have spoken truthfully and righteously). Then, one should send blessings upon the Prophet (peace be upon him) and ask Allah for Al-Waseelah for him.

It is also recommended for the listener to say upon hearing the Adhan: "Wa ana ash-hadu an la ilaha illallah, wahdahu la sharika lah, wa anna Muhammadan rasulullah, raditu billahi Rabban, wa bil-Islami dinan, wa bi Muhammadin sallallahu alaihi wa sallam rasulan." (And I bear witness that there is no god but Allah, alone, without partner, and that Muhammad is the Messenger of Allah. I am content with Allah as my Lord, Islam as my religion, and Muhammad - peace be upon him - as my Messenger.)