Tom, I certaintly don't claim to understand the communication skills of our CDTs. I must note that our two older tortoises can be deep in their burrows, well out of visual and possibly out of olfactory communication with us when they come a-charging toword our location. Our large outdoor enclosure is primarily decomposed granite, sand and clay. The surface we approach from is concrete. How they can understand our presence, I don't know . . . but they clearly know that we're coming . . . without a visual line of sight. Olfactory sense would have to be able to quickly move from our patio to their burrow. My very rudamentary knowlege of smells, lends me to believe that traveling down, away from convection waves, says that this would be difficult. I'm eager to see where this thread goes . . .@GBtortoises shared the results of a scientific study done at his place years ago. The conclusion was that tortoises don't hear and that their reaction to humans is olfactory or visually based. Another possibility is that they can recognize the "feel" of an individual's foot steps, especially when paired with a primary reinforcer like food.
After reading his posts, I tried to call some of mine by name, but they always seemed to be able to see me before I could try voice only. They have such a wide field of vision, its hard to sneak up on them. So far, I've gotten no responses to voice calls.
I find this to be an interesting and debatable topic.