Here’s what I think it means:
Successful verbal communication depends both on the speaker’s choice of inflection and intonation and on the listener’s understanding of the significance of the speaker’s choices. Verbal communication is thus prone to failure if, for example, the speaker makes inappropriate choices (deliberately, as in an attempt to deceive or confuse the listener, or accidentally due to illness or stress) or if the listener is not familiar with the speaker’s social and/or cultural conventions. By carefully choosing unambiguous grammar and logic, the speaker can convey meaning without risk of being misunderstood.
This is especially important in circumstances where face-to-face communication is not possible, as in written communications or telephone calls. It’s also critical in circumstances where the listener does not have the opportunity to ask for clarification.
Here’s how I might have expressed it:
The effectiveness of direct, i.e. face-to-face, communication relies as much upon the speaker’s choice of intonation as it does upon his choice of words; indirect communication is made more effective by careful attention to grammatical and logical construction of statements.