In defining your canine's voice, I suggest you first consider the relationship dogs have with humans. To me dogs typically have a submissive relationship and seem to want to placate their owners. So is your canine character going to take that role or have a more dominate personality. You didn't define whether the dog is the MC or a supporting character and I think that would make a difference. I suspect the dog is a supporting character, so how will it interact with humans? Does breed have a bearing on voice? I would think the dog's personality and voice would be different for a small, nervous breed and a large aggressive breed. Is the dog expressing personal thoughts and feelings or representing the canine world?
That's tricky, because the only human in the story who can hear the animal's telepathy is the dog's supposed 'master', so the only time you 'hear' the dog is when he's lecturing his human. On the other hand, the dog is the current leader of the animal movement, as he knows more about humans, how they think and what they know, that the many pets who have no clue what humans are actually talking about. What's more, the entire time he's been living under his human family's roof, he's been orchestrating a 'revolt' to replace humanity. In the end, even though he's not supposed to, he brings them along because he can't abandon his pack, which leads to a continuing sense of conflict for the few humans, so continually have to prove both their value and their non-threatening actions.
Of course, the reasons for all of these stances are the substance of the plot, so I can't really reveal them without giving away much of the overall story.
But since the dog in question, Bowser is his name, doesn't talk often, it's usually to explain fairly complicated 'facts' to his former human owner, who then puts it into words for the skittish humans, also expanding on the scientific validity of what he's proposing, as a way of justifying what he's saying. Thus the dog has be extremely knowledgeable, even as he needs to 'sound' non-human (different cadence, different manner of speaking and acting).