If you have voice mail and someone calls your Virgin Mobile phone and gets your voice mail box the caller gets charged for the call. This is rightfully so as the caller can leave information for you.
Virgin is now instituting a new policy that if you have had voice mail removed from your account, Virgin Mobile will send a supervisory signal back to the CALLING TELCO that your Vrigin phone has answered. That means, that if someone calls you from a pay phone and you do not answer, and you do not have Virgin voice mail they will get an intercept message that could cost the pay phone user fifty cents each time [depending on charges in your local area].
Virgin has already started this and has stated that eventually all their customers will have this "supervised as answered".
Perhaps they don't like lost revenue if you do not have an active mailbox.