This type of service on a PBX is called DISA, or direct inward system access. It allows a person to call a specific number assigned to the feature, the system answers, the caller inputs a security code and is given access to outgoing or internal calls or even other feature activations.
There are also stand alone devices that do this with two lines. A company called TELTONE Corp has made one for years. I think the model is M-106E. It has several features and works quite well. I don't have their URL at the moment, but their phone number is 800-426-3926.
Hope this helps!
....JIM....