As exsmogger said, that ext is probably assigned to mb 100, but that was probably done for a reason...
First, what model VM are you using? On a call pilot, you can disable the general delivery.
MB 100 is where calls go in VM as a "last resort"(If a caller presses a wrong digit, or is transferred to vm from an ext without a mailbox, etc...).
If you do decide to change the assigned mailbox on x221, I'd definitely find another set to assign mb100 to, otherwise your customer is going to miss calls.