Ships provide cell phone coverage - but it costs about 2.50/min. You are using their satellite connection, not a cell or your providers sat. I'm not sure how much texting might cost. You should have documentation about this in your cabin.
Enjoy your cruise.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
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You will be too far out to sea to pick up any land based towers. Actually the ship, automatically, turns on their MTN cell system once any land based signals become weak enough. Even if a land based signal becomes available for a bit, while at sea, the ship's system will override it. You will be able to use your cell phone but, as Green said, it will cost you around $2.95/minute.
The ship will turn off its system when they are in port so you will be using the local carrier's signal. Make sure you call your provider and add "International Roaming" to your plan to make sure you have service on the ship and in your ports. There is no charge for adding this feature.
Take care and have a great cruise.
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