Orthodox church isn't as homogenous as the Roman church. Orthodox church had several patriarchies in the middle ages, each responsible for its country or region. So calls from religious leaders from Orthodox churches wouldn't have as much impact as Popes calls for crusades were. However Byzantine Emperors, Serbian and Bulgarian kings did call for crusades against the Ottomans with more or less success. You can read about it in G. Ostrogorsky's History of Byzantine Empire.