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What number is it that's calling so that Vodafone can make sure it's legitimate and look into it for you?
Another option would be to register the number with TPS (Telephone Preference Service). It's done wonders for me with nusiance calls since signing up.
Vodafone customer since 2004. Attempting to help where I can on the Community
Is it definitely Vodafone who are calling you? I'm not aware that this is something they would do and it sounds more like an independent seller. The best thing you could do would be to block the number.
If the number that's calling you is displayed, Google it and see if it leads back to the source.