To use Skype, you would most likely need the ship board WiFi package then use the skype app to make the call. I have done facetime from the ship using the ships wifi.
The cellular at sea IS expensive and unless your work covers the bill to be reached 24/7, i would not use my phone on the ship. You can get an international plan and use your phone at the ports but again, its usually not less than 2.00 a min if not 2.99. We use the wifi in different places if we need to speak to someone and again, use the facetime app. When in St Thomas or San Juan...our service from Sprint is covered as they have service in both places.
Previous Carnival sailings:
Carnival Sensation 2013
Carnival Liberty 2013 (Eastern)
Carnival Liberty 2013 (Western)
Carnival Victory 2012
Carnival Triumph 2001