I think they will be able to help you as if you look at the terms and conditions,
18.1. You authorise us to use and disclose, in the UK and abroad, information about you, your use of the Service (including, but not limited to, phone numbers/email addresses of calls, texts and other communications "Communications" made and received by you and the date, duration, time and cost of such Communications)
Im not sure if they will help you, but they keep track of your phone usage so they should know the numbers of those calling you on a private no, so in theory, they could block them. It may be a lot of work but its possible.