Mordor Communication

[Bingo]

‘No, I don’t know,’ said Gorbag’s voice. ‘The messages go through
quicker than anything could fly, as a rule. But I don’t enquire how
it’s done. Safest not to. Grr!

This intrigues me. How exactly do the messages go through? I assume he's talking about messages from Minas Morgul to Barad-dûr, or else he'd say they "come through." It seems like it could not be by palantir, unless Sauron secretly has two of them.

What other means of communication could there be? I can't help imagining a secret telephone network connecting Sauron with different parts of his dominion. Or might the Nazgûl be telepathic?