You will receive instructions in your documents but the number I had from my Oceania cruise was:
1-866-242-7447 (1-732-335-3277 for calls outside the U.S.)
The person calling needs to have a credit card ($8.95 per minute) , along with the ship's name, passenger name and the stateroom.
The caller will be charged for the call and not your on board account.
You can also use your cell phone and have a predetermined time for someone to call you or you can call them. It will $2.95/minute for the call.
The least expensive alternative, for you, is having the person use the 866 number and credit card and call you.