Are you sure this is a voicemail feature? Camp-On, and call back are typical PBX features. I don't recall that the Octel systems have this feature, (but I could be wrong)
With the Camp-On feature of a PBX, it works as you discribe, if the phone is busy, the PBX prompts the caller to Camp-On, it's like a exteneded call waiting. The called party then hears a "camp-on" tone to let them know someone is waiting for them to end the call. This is typically used by an attendant, who transfers a call to a busy extension.