Hi ​@LindsayJ 

You should not be charged for data roaming in France as it is included in iD Mobile plan's roaming allowance, up to 30GB per month. If you are experiencing unexpected charges, it's likely due to one of the following reasons: roaming on a ship(#) or other location not included in the 50 free roaming destinations, exceeding the 30GB data limit, or not being properly connected to a French network(#). 

(#) -  In certain areas you need to be aware if your phone is connecting to a maritime network before use. You can be in a ‘free to use’ roaming area e.g. France, but may have inadvertently connected to a non-roaming network in certain places e.g. in coastal areas.
Maritime networks use satellite connections, which are more expensive than terrestrial networks. 
To avoid unexpected charges, it's recommended to disable data roaming on your device when near the coast and switch to Wi-Fi when available
This happens when your phone connects to a maritime network because it's searching for a cellular signal and automatically connects to the nearest available network, which could be a satellite-based network on a ship or at a port.

If you connect to the network out at sea, known as the maritime network, you may incur large charges, please find out how much below:

https://www.idmobile.co.uk/help-and-advice/international-and-roaming-charges

However, to avoid these charges, you’ll need to ensure you set your bill cap via the iD Mobile app/website to £0.00, which will block all charges, and from being able to connect to the maritime network.

To further guarantee you don’t connect or get charged, you can turn your network off via your phone’s settings, or even go onto airplane mode.

Hope this helps x

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