For us, it is more of controling who can make a call. For example, you have one site (SiteA) that is not allowed to call a certain number, say a 1-900 number. You have a second site (Site B) that can make that call and the two are connected via a T1. If a person at Site B calls a 1-900 number, then transfers the call to Site A, you in effect have bypassed the calling security for Site A.
Keep in mind this would require some coordination, as well as a strong desire to do something against policy, but most rules and policies were put in place because SOMEONE decided to do something that others found to be wrong.
Not a big deal, but as I said, someone decided to do this at some point in time, and now we don't allow it any more.
Scott M.