Who is the Mu'adhdhin in heaven, if one exists, and is it true that Bilal ibn Rabah was the first Mu'adhdhin in Islam?

Bilal ibn Rabah (may Allah be pleased with him) was the first to call the Athan for the Prophet (peace and blessings of Allah be upon him) in Medina. As for the Muezzin in the heavens, there is no authentic report from the Prophet (peace and blessings of Allah be upon him) concerning him.