Alright this gets more puzzling. I've just configured my callcentric account on Line 2 by following the guide at
Linksys PAP2 configuration. And guess what! Incoming works perfectly for call centric. I've tried calling through SIP Broker as well as direct SIP uri (xxxxxxxxxxxx@in.callcentric.com) and both of them work perfectly. No port triggering! No port forwarding!
So, the issue I am facing, is this specific to Voxalot+PAP2? Should this be raised as a bug somewhere? Could someone kindly confirm?