Yup. I did that. Of the available options, the one that worked best was Blacklist. The caller hears something like "You cannot reach the number as dialed. Please check the number..."

The Busy Signal did just the opposite of what I'd expect. The Voipo phone rang, but upon being answered, it produced a busy signal. I would have figured that the calling number would hear the busy signal and the Voipo phone would do nothing.

In any case, I highly recommend the "Blacklist" option.