That's not possible. In which iPhone is your daughter using the eSIM of her mobile subscription (Start M) ? When I checked yesterday, this was linked to an iPhone 6S. Now I see an iPhone 13. Do you both have an iPhone 13 currently?
OneNumber is an extension of the subscription. So you need to order this for the mobile plan which is going to use this. You and your daughter have 2 seperate subscriptions, you Red Unlimited and your daughter Start M. Both subscriptions are active under 1 customer number (this also means 1 My Vodafone account).
If your daughter wants to use OneNumber, she needs to order this on her iPhone 13 with her own simcard (eSIM). You can't order this for her (on your phone) with the simcard of your own mobile plan, because you only can order OneNumber as extension of your own seperate mobile plan.
Your daughter needs to follow these steps:
- Open the Apple Watch app on her own iPhone
- Go to 'Mobiel netwerk' and choose 'Configureer mobiel netwerk'.
- Login with the details of My Vodafone and choose 'Order OneNumber'.
The Apple Watch you are going to use needs to be a cellular watch (you can recognize this by the orange or red on the crown)
Additional information: With the error: The vf nl acount on this iPhone cannot be used to add a mobile plan to ...watch" it's important to check if the iPhone and Apple Watch have the latest software. It is also important that your Apple Watch was purchased within the EU. Was the Apple Watch purchased outside the European Union? It may not work because it uses a different frequency.