With the sun high in the sky, Ayubur Rahman gives his weekly call for the Juma, the Muslim congregational prayer recited midday each Friday. His melodic voice rings out over a Roxbury neighborhood from a loudspeaker atop the Islamic Society of Boston Cultural Center, where he is the muezzin, or the one designated to summon Muslims to worship. “God is great,” Rahman begins in Arabic with a singsong inflection. “God is great.” The call to prayer, known as the adhan, greets people of many stripes as they stream into the mosque — they are old and young, cabdrivers and professionals, men in suits and women in abayas, representing many countries and cultures. They have come together to pray and to hear the imam deliver his weekly khutba, a sermon. Though Friday is special, the call to prayer happens five times daily, in a schedule determined by the sun’s position. With the exception of the early-morning recitation, the adhan is broadcast outside the mosque, near the corner of Columbus Avenue and Malcolm X Boulevard, its spiritual invitation floating above the Roxbury bustle.