I think it calls their servers when creating a route and downloads the required maps in one shot. I've both made and received calls while navigating just fine. Also, I've received SMS and Exchange pushmail while navigating. I'm very pleased with Telenav right now. Only (minor) drawback is that off-route re-routing takes a few seconds longer than having maps loaded since it's another data hit to their servers. OTOH, Telenav is smart enough to keep navigating via the programmed route and "predictive" logic so if you emerge from a tunnel, for instance, the program has a pretty good idea where you are.

Todd/Indy