As suspected, Openreach cannot deal with this type of request via the number suggested, nor any phone number. After speaking with several different people / departments, the route for requesting this be dealt with is:
Send an email to network.rearrangement@openreach.co.uk with the subject "Urgent rearrangement of cables". Include your address, contact details and a description of the issue with a What3Words location for the cable. There may be charges associated with this work which you may be liable for, depending on the situation.
