We found there was no signal whilst we at sea.
We found it variable - sometimes we had signal, sometimes not.
Take care. We were on Island Star 2 weeks ago and while at sea my mobile connected to a US based network! Fortunately my Virgin mobile always sends a greeting text message giving details of charges and it quoted £1 per minute. I don't know where the signal came from (we were well out at sea) but Island Star doesn't have a ship based service. Possibly a US ship with mobile capability was nearby. If you are in sight of land you may get a signal, it depends on the strength of the land based transmitter.