I don't know...I grew up believing Han just threw on the helmet and made the call. Plus, he gives that little shrug and smile when the Imperials run out and get duped, so at the very least he was behind it. It really does sound like his voice though. Honestly, it's the easiest and most likely explanation...helmet, hand in front of face, call.
As for the driver thrown off the AT-ST, I'm wagering broken neck at the least. You don't get tossed by a Wookiee off a 28ft tall moving vehicle and just get up and dust yourself off. Plus, there were a ton of pissed-off Ewoks just waiting on the forest floor.