[Gurthang]

Sauron could communicate with them somehow, but I don't think it was telepathy. Also, they obviously could not know everything he wanted, or else they would not have needed the signal that told the Witch-king to start his assault on Minas Tirith.

That signal is what really throws me off. For a while I thought that maybe they knew so much of his will that they always knew what he wanted of them. That would explain how they were slaves to his will. But the signal throws that theory out the window.

Perhaps their rings had some link to Sauron, so that He could will them where he wanted, but could not tell them what to do.

Or maybe they did have some mini-palantiri.