It's not peculiar to Volkswagen. Alot of dealerships do this now.
In the vast majority of cases when verifying garages, I've found the geographical number(s) listed on
BT Directory Enquiries.
It is also commonplace for the vehicle manufacturer to give the dealership one 0845/0870 number and the dealership to show a different 0845/0870 on its website. This is probably more to do with gathering statistics.
One example is
Perrys which have bucket loads of different 0845 numbers and frequently promote different 0845s in different places (newspapers, website, flyers etc) for the same garage.