What is the legal permissibility of the call to prayer (adhan)?

Linguistically, the Adhan means notification, and Islamically, it is the notification of the time for prayer. The Adhan was legislated in Medina after a vision seen by Abdullah ibn Zayd, where an angel taught him the Adhan, and the Prophet, peace and blessings be upon him, affirmed this vision. The Adhan is a communal obligation (fard kifaya); if it is left due to forgetfulness or ignorance, the prayer is still valid. It has been reported concerning the merit of the Adhan that everyone who hears the muezzin's voice will bear witness for him on the Day of Resurrection.