I think you will need the VM forum staff to look into where the block lies.
VM have anonymous caller rejection (ACR) available as part of their service but you have to buy into that and pay extra per month for the service.
Have you tried a test yourself by using another phone to dial into your landline and prefixing your number when dialling with 141, which should withhold the number you are calling from just for the one call? Then see what message you get when calling in. That would confirm you have a general withheld block in place.
Also worth double checking any settings on your landline phone handset. A good many of these do have the option to block withheld numbers on the handset. Might be worth checking something like that isn't accidentally turned on by mistake.
The VM forum team normally reply within a week and are usually quicker than that.