Welcome to Vodafone Community
The Apple Watch shares your minutes, texts and data with your pay monthly iphone plan. Please see this link to see if your plan is eligible to be shared with Apple Watch.
Please also see this link for Vodafone One Number
The best way to get this solved for you is through the Customer Service channels where an advisor will have the necessary account access and will be able to advise accordingly. The is a live chat connection on the first link in my reply.
Hope this helps.