They said it was urgent I contacted them as my NI number has been blocked as it was used fraudulently. Voice mail left saying to contact them urgently and not to ignore this message
Claims to be from "Her Majestys Revenue Service" or something. Notifying that your National Insurance Number has been cancelled due to criminal activities and to make contact. Load of $#! T£
Voice-simulated message claims that your national insurance number has been terminated and demands that you call back on this number.
(Presumably it's some kind of premium rate number.)