Well, potential spoilers, but Carl's route actually has nothing to do with the "main" storyline. It's mostly just an info dump on the past, what's already happened. That said, it's still a great route.
Also, the backstory of the "voice" is explained in Jenna's route, not Carl. I recommend doing Jenna's route last because her good/happy ending is the best note to end the game on, and wraps up Echo nicely.