Hi @crambie,
Did you receive a "welcome" message to advise of roaming rates, or a notification that you have incurred a roaming charge?
It's unusual but not impossible to pick up French signal from Dover, I've also seen cases of phones connecting to maritime networks in coastal areas when near ferries or cruise ships. If you haven't already done so, switching off roaming data in your network settings can help prevent accidental charges.
Thanks,
JonathanB
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