Viking offers free Wi-Fi, and AT&T allows Wi-Fi calling. On our previous Viking cruise, we had airplane mode on for the duration of the trip including excursions and at sea. We were able to talk, text, and use data the entire time on our phones and PC when we were on the ship with not a single charge although responses were delayed sometimes. We were in a travel group, and we could communicate with each other via cell phone, and we could make and receive calls to the US for no charge. Audio quality was normal. All this was when we were in range of the ship's Wi-Fi.
I believe airplane mode turns off roaming, data usage, and maybe some other stuff, but that is the key to managing international fees without an international calling plan.
John