In Harry Potter and the Philosopher's Stone (or Sorcerer's Stone), Hagrid Tells Harry he "flew" when asked how he'd gotten to the rock, but the two take the boat back to the mainland leaving the Dursleys on the rock in the little shack without transport back to the mainland themselves or Magic to help them get any.
The place is described as being so out of the way that I don't imagine cel service was good - Not that Dursley is ever described in a way that makes you think he had a phone with him to begin with.
So, first, how did they get back off the rock? and second, how is it they didn't kill Harry when he returned to them after his trip to Diagon Alley? After-all their son now has a tail and their experience trapped on the rock must have been harrowing.