But a substantial fraction of calls are made to cellular phones. It was done in the early days, before the number of cells and customers made it impractical. The alternative to flood paging is registration-based paging. That's where the phone announces itself to the system with a short message on the access channel so that the system knows exactly where to direct a page should an incoming call come in. If the mobile moves to another cell, it re-registers in that new cell and the system updates its database accordingly.
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.