Strangely, IMO, VM have to activate/deactivate voicemail for customers.
Not sure from your description if yours is actually set up or is set up and not working properly.
I believe the maximum number of rings is 10 before answer. VM also have to change that for you too if you want it setting to 8 rings.
One of the VM forum team will normally reply here within a day or two but may take a bit longer due to the bank holiday weekend.