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Call 0800 800 150 (or just 150 from a BT phone). You should be able to get it changed but there's likely to be a charge, £25 the last I heard.
If the calls are abusive you can report them to the police. If you are given an incident number I think you can get the number changed for free. See this recent thread https://community.bt.com/t5/Phones/Changing-number-at-police-advice/td-p/1350283