I don't believe I have ever even seen him speak. Indeed, in Hesiod's Theogony, one of our earliest sources from ancient Greece, we see the following,
Hesiod said:
"Typhaon the terrible, outrageous and lawless, was joined in love to her [Ekhidna] ... And next again she bore the unspeakable, unmanageable Kerberos, the savage, the bronze-barking dog of Haides, fifty-headed, and powerful, and without pity."
So apparently he barks, not speaks. Also:
Hesiod said:
"And before them, a dreaded hound [Cerberus], on watch, who has no pity, but a vile stratagem: as people go in he fawns on all, with actions of his tail and both ears, but he will not let them go back out, but lies in wait for them and eats them up, when he catches any going back through the gates."
So that's sort of wily, but it is mostly just an animalistic strategy, and this is the smartest we see him acting in the following sources:
Homer, The Iliad
Homer, The Odyssey
Hesiod, Theogony
Greek Lyric IV Bacchylides, Fragments
Apollodorus, The Library
Aristophanes, Peace
Aristophanes, Frogs
Euripides, Herakles
Callimachus, Fragments
Quintus Smyrnaeus, Fall of Troy
Strabo, Geography
Pausanias, Guide to Greece
Diodorus Siculus, The Library of History
Plutarch, Lives
Hyginus, Fabulae
Virgil, Aeneid
Ovid, Metamorphoses
Propertius, Elegies
Cicero, De Natura Deorum
Valerius Flaccus, The Argonautica
Statius, Thebaid
Statius, Silvae
Apuleius, The Golden Ass
Mostly he is tricked, or assuaged by sweet cakes (sometimes these are drugged, like in the Aeneid).
I would guess the intelligence is an artefact of 1e's Deities and Demigods then, but I may have missed something (though if I did, it was probably more apocryphal than the sources I included). Hope this helps!