Different carriers have different registration policies. I know from personal experimentation with GTE in San Diego that one's phone must successfully register before it can receive a call. If you set up this feature and then call your cell phone when it has been switched off for a while, the call immediately forwards. But switch the phone on, let it register, turn it off and then try calling it. There will be a much longer pause while the system unsuccessfully attempts to page it in the cell where it last registered, and only when this fails will the call forward.
Most phones give no audible or visible sign that they're registering.
The IN USE indicator remains unlit even though the phone may be actively sending registration messages. But this is clearly an unintentional artifact of this particular design. To make sure, remove the battery pack. Naturally, a registration process is involved in providing for call delivery to roaming mobiles. The protocols are different because multiple carriers are involved, but the principles are exactly the same.