FreePBX has the following two different modes that can be used in order to configure extensions:
Extensions
DeviceAndUser
In the Extensions mode, each numerical extension that is configured within FreePBX corresponds to a physical telephony endpoint. An endpoint could be a physical telephone, a softphone, an Analog Telephony Adapter (ATA), a speaker, or anything else that talks to Asterisk. In this mode, each endpoint is statically assigned to a particular device. This device never changes.
An example of this would be a typical cubical-office environment. Each cubical has a phone and the extension that rings the phone will never change. Even if the employee who works in a particular cubical is replaced, the extension that rings their phone will remain the same. Most end users find this setup intuitive, as it allows them to dial one number and their call to ring one phone. This mode is the easiest to set up, but offers less flexibility than the DeviceAndUser...