The invoice states calls whilst abroad, so you were indeed on the French network and as such these calls (whether made or received) are not included in your plan, hence the charges.

The French operator has not provided Salt with the exact number you dialled or your phone has dialled a premium rate service.

I would email Salt and ask for an explanation of exactly what the calls are. They must reply to your written request.

Did you make or receive any calls at those times (regardless of the number cited on the invoice)?