Have you ever tried to call your own number? Not your number on another phone, but the one on the phone you are holding.
Did it ring? I bet it didn’t. I have tried this a few times myself, and the result is always the same.
The technical explanation for this is that when you start to call the number, your phone sends a signal to satellite that it’s busy (currently in use). Due to this, it can’t receive another signal since it’s already sending one, and that’s why it brings out the error message (busy)