Per the Scripting Guide:
"The Controlled keyword only works if the ultimate destination (directly, or indirectly through switch features such as Call Forward) is an internal DN (that is, on the same switch). If the DN is an external number that cannot be connected (for example, because it is busy), the Controlled option can be ignored. If this occurs, the caller receives a busy tone, and the script stops processing the call. . . . Controlled mode applies only if the destination is an internal DN that is busy or does not exist.