Chris

Yup ... You're suffering what Openreach would call poor pair quality. On their network it could be caused by water ingress, corrosion, erosion of the insulator, bringing the wires into contact. Don't know what the most likely diagnosis is on a VM phone line, though I understand they rely on a copper pair as far as the street cab, so maybe that needs replaced.

You will have to phone them to get someone out, as it is nothing you can fix and it won't fix itself.