Again, the paging channel or path is another word for the forward control channel. It carries data and is transmitted by the cell site. The MTSO then assigns yet another channel for the conversation. But I am getting ahead of myself. Let's finish registration.
Registration is an ongoing process.
Moving from one service area to another causes registration to begin again. Just waiting ten or fifteen minutes does the same thing. It's an automatic activity of the system. It updates the status of the waiting phone to let the system know what's going on. The cell site can initiate registration on its own by sending a signal to the mobile.