Translation:They will have called the restaurant already to make the reservation.
"They probably (they must have) called the restaurant to make the reservation"?
Danielconcasco is right to point out the matter of tense, but i agree with you regarding "a reservation" rather than "the reservation". That is how native english speakers speak, perhaps because to us "the reservation" only exists after having made "a reservation". That is unless it is a question "Have you already made the reservation?