Keep in mind that the global phones you refer to are satellite phones. There are no cell towers at sea. The satellite phone needs a clear shot to the sky to operate; you need to be on a verandah or open deck with it. By all means get one if you'd like that option.
However, you may wish to simply take your regular cell phone along, and use it while you are in port on days 3 through 6 to check in with your family. On Day 2 and Day 7, while you are at sea, leave it to your family at home to have handy the ships telephone number so that they can call in the event of an emergency. This can be supplemented with onboard email once or twice a day.