Most Demon Gods in the universe did not communicate through speech. They were not carbon-based lifeforms and had no organs called mouths. They generally used a kind of frequency humans could not understand, while human language sounded to them like another peculiar frequency.
The mages of Kamar-Taj could receive the sounds of the universe through meditation—that was, they could hear the frequencies used by the Demon Gods to converse. This allowed them to communicate smoothly with the Cosmic Demons. Of course, some especially reclusive Demon Gods never joined the cosmic chatter. Their way of broadcasting frequencies was clumsy and difficult for mages to understand—the equivalent of a country accent.
But even a country accent was still speech. Yet for some reason, the gods in Africa mostly did not know how to talk.
At first, Strange had not understood what the mage meant by "unable to speak." When he had first begun training, he had not understood the frequencies of the Cosmic Demons either. He could only force out words one at a time when communicating with them, and anything more complicated required translation.
The Sanctum Sanctorum had been in business for so long, and Strange had not been idle. By now, he could basically understand the frequencies of Demon Gods from every region and even join the universe's conversations when he had nothing better to do.
But when he heard a deafening lion's roar coming from the other end of the phone, he truly understood what it meant to say that the universe was vast and full of wonders.
This lion god really could not speak. He could only roar—very loudly—and no one could understand a word of it.
Strange covered his forehead and called Schiller. "...Are you sure that situation is life-or-death? We've run into a small problem here..."