Seriously, they just look like their owner. For example, Robin talked to Zoro through one in Dressrosa, and even the snail had a scar on its left eye. That is just weird. We even saw Kaidou talk to Scratchmen Apoo, and we know because the snail had braces. Trafalgar Law contacted Donflamingo through one as well, and this one had a hat just like his. Let's not forget Doflamingo himself had one for himself in Vergo's pocket, which was also wearing pink sunglasses.
A long time I came to the hypothesis (because I read the manga only) that with the Den Den Mushi you don't hear the actual guy's voice, but rather, the snail can imitate it perfectly to you with his facial expressions so that it is like an actor, with face and voice. Am I right?
Now I am starting to think my hypothesis was wrong because of this similarity thing.