When you reset a 7921 VoIP phone to factory defaults (Toolbox > Phone Settings > **2 > Yes), the phone automatically looks upon rebooting for the SSID cisco, open authentication, no encryption.
Whatever the phone configuration is, it first behaves like a standard 802.11 client, it scans channels and looks for the SSID it is configured to use (cisco and/or any other SSIDs that you may have configured in the different profiles that may be enabled at boot time).
Once it has found a valid SSID, it tries to authenticate/associate. On the phone screen, you see "Locating Network Services".
Once this step succeeds (the phone associated successfully), the 7921 needs to get an IP address, usually through DHCP (you see "Configuring IP").
Along with the IP address, the phone should get the IP address of a TFTP server (this is called option 150 in the DHCP scope). The phone needs this TFTP server to obtain its configuration file and verify that its firmware is the right one. Usually, the TFTP server is directly the Call manager or Call Manager Express, but it could be a standard TFTP server.
Once the phone has its configuration file, it reads from it the list of Call Managers (Express or not) that it is supposed to try to reach. It contacts them one by one (you see "Configuring CM List" on the phone screen).
The nice thing about using the Call Manager (CUCM) or Call Manager Express (CME) as the TFTP is that finding the CUCM or CME is easy!
Once the phone successfully found a CME or CUCM, it registers to it (you see "Registering" on the phone). This is the step where the 7921 gets its extension number.
The phone is then ready to place and receive calls.
When you configure the 7921 from its screen, most features are locked. You unlock them by keying **#. As soon as you key these three keys, a new soft button appears, displaying "Change" or "Edit".