Mobile phones on P&O cruises

We're going on a P&O cruise soon. For my phone I use Giffgaff which uses the 02 network but at a fraction of the price. Giffgaff doesn't work with the system used on the ships.

Does anybody know whether Tesco mobile does. I won't want to use the phone much when we're at sea but I love surprising my parents with a call from the middle of the Med.

Thanks. Angela.

Welcome to Cruise Critic. Best ask this on the board here for P&O-UK, under Cruise Lines P-Z. EM