But the amount of noise that each call adds to the noise floor is not constant. It's a function of the location of the phone and thus how much power it needs to use to get its signal through to the cell, and it's also a function of the actual voice traffic it is carrying, which varies from moment to moment. This leaves 61 for traffic channels . In practice, however, no cell ever can support 61 calls simultaneously. But exactly how many they do get is very much dependent on circumstances, and it changes from second to second.
Many factors feed in.
So when the cell system tries to create a new call, it doesn't actually know whether the cell can sustain just one more. What it knows is which zone you are in, but a zone usually consists of more than one cell, usually having more than one sector each. Any time your phone moves from one zone to another it has to register , which means that it turns its transmitter on and sends a brief message to the cell system announcing some important information about itself. The cell system knows where it received that message, and thus at that moment knows which sector of which cell your phone is locate within. However, thereafter you can wander at will within the whole zone and the cell system won't have the faintest idea where within it you are.