From what we were told, the person calling has to have a credit card because the charges are applied immediately -- it is $6.95 and I think there is tax added onto that!
What we did last time was leave our email addresses with friends and relatives. For my mother, who is 82 and no way computer literate, I left her my girlfriend's phone number. If she needed to reach us, my girlfriend could email me on the ship. We checked our emails daily. That also went for the kennel and person taking care of our other pets -- they all had our email addresses.