The forward French

Yesterday I was standing on the beach in Dover with France barely visible 30-odd kms away through the haze of the English Channel. Suddenly my cellphone beeped with an sms from a French cellphone network welcoming me to France! I presume this is because the French signal is stronger than the local UK signal. The Brits must be highly offended by this French invasion of their airwaves, particularly the people who live in the area. If I made a call would I have been charged an international rate? I wonder. This problem must be so much worse in continental Europe where you're never too far from a border and a potentially forward neighbour.