Hello Kevin,
You can transfer calls to other extensions (from AA) which are not having voice mail box enabled for them by checking this option "allow Transfers to Numbers Not Associated with Users or Call Handlers ".
And, suppose you have 2xxxx extension range then you can unblock/uncheck pattern like 2???? in the restriction table to reach 2xxxx extensions, this will not allow to transfer calls to unwanted numbers (toll fraud). OR, having the CSS for Unity Connection is restricted from PSTN toll access may also be a good option.
Suresh