I would disagree with the person above me. "Called to prayer" is a phrase that is definitely used in English. "Called to pray" would certainly make sense, but I've heard the first option far more. I would think of "prayer" being used almost like a location or a building and the "to" being used like a preposition (like to the hospital, or to the mosque, perhaps in this case.) Of course, there is no article in front of "prayer", which I really can't explain. I think this is another weird use of the English language... It also could be Arabic phrasing translated into English. I studied Religion and "call to prayer" is a concept used often in many faiths. Maybe this could help you a little on the context: https://www.thoughtco.com/what-do-the-words-of-the-adhan-mean-in-english-2003812

I'm a muslim from Malaysia. Usually we say "called to prayer" because prayer we refer to action (to perform the muslim prayer). "To pray" we refer - to ask anything from God in our heart.

It should be "they are called to pray", you are right. That "prayer" must have been a typo.