Although it's not very clear, it's still accurate.

It's possible for the caller not to have an inclusive plan.  They could just be 'weekends' only or e&w so therefore it's posible to have to pay for the call.

As for the mobile bit, we all know calls from mobiles to geographical (never mind 03) are more expensive compared to landline providers for those that dont have inclusive minutes (payg for example) or have gone over their inclusive minutes allowance.

I assume you were comparing 03 with 01/02 but the ad could be more generic and informing everyone that calls may differ depending on where called from (landline/mobile) assuming caller not on an inclusive call plan.

Or it could be that someone's got their wires crossed and put on "mobile costs may be higher" because that's commonplace today on many adverts with 0845 numbers.