Ringing someone on board ship costs an arm and a leg to the person ringing and often to the recipient but you may find this acceptable in the circumstances and they will be informed of a contact number for QM2 or you can go to Cunard’s web pages and find out there. An alternative is for them to set up an e-mail account on the ship and for you to let them know that way should the happy event happen whilst they are at sea. If they are close enough to land you could also get them to ring you on their mobile now and again ... Nei.
We cruised on the QM2 in March this year from New York to the Caribbean & back. Keeping in contact with family & friends was easy. When you receive your tickets you will be given your own personal e-mail address. When you are in the privacy of your own cabin you just use your remote control for the tv & go into your own personal e-mail address. You are just charged a nominal fee for both your inbox & outbox. If my memory servers me correctly, I think it was about $1.50. per e-mail.