Is this "silent prayer" a figurative meaning? Or did they do something almost silent compared to "silent prayer"? What does it mean?
ex)Perhaps the best news about speaking to groups is that it is a learnable skill. Most people who are competent speakers today could only say a silent prayer in a phone booth at one time.
It just means that people who are now very competent at speaking aloud to large groups might, in the past, have been only confident enough to stand in a phone booth (ie alone, not in front of a group) and say a silent prayer (ie they didn't even have the confidence to speak aloud in front of themselves!)
It is possible to learn how to speak aloud to large groups, it is not necessarily something which comes naturally to some people.