I'm not sure this is specific to Delta. I have observed that cultural norms vary by nationality. Where some might blanch at the idea of taking a call at a conference table/business dinner, others will proceed without hesitation. The requirement for headphones on-board is something I welcome but I don't think gate areas are ever going to be quiet - CNN Airport Channel and the payment streams to airports authorities killed all hope of that. As for on-board behavior - hey, at least Delta prohibits calling in-flight. I carry some emergency noise cancelling headphones.