Press the Setup button: Navigate to Menu Item #14 - Factory Reset, and press Select: Select OK at the Reset Warning... Commencing Reset and Initialization... (lots of blinking lights...) The SPA504G is now reset to its factory default values.
Following a Factory Reset, any passwords previously set have now been removed and will need to be re-set, if desired, later. (Not to mention that all previously set user configuration data is also lost)
Now, we have a clean slate from which to begin re-configuring the SPA504G IP-Phone.
This will be a good time to check your Firmware Software Version, and compare it against the latest available firmware on the Cisco support web site. I explain how to check and update firmware in another post found here.
Now, you should also check and confirm the IP address assigned to your SPA504G by your DHCP server/router. You will need to know the IP address in order to log into the SPA504G web interface via your LAN side PC's web browser for the configuration procedures.
Checking The SPA504G Assigned IP Address
Before proceeding here, you will want to be sure the SPA504G is connected to your network LAN with an Ethernet cable. Following a factory reset, the SPA504G is set, by default, to receive its IP address via DHCP server via its Ethernet port, marked SW (switch).
Navigate to the Setup Menu item #9 - Network, and press select. Take note of the IP address shown here for reference.
If you will be Port Forwarding SIP and RTP ports to your SPA504G, you may want to turn off DHCP here and set a fixed IP address. However, if you will be using Port Triggering, then you should be fine with DHCP assigned IP addresses.
Now that we have successfully performed a factory reset, we are now set to configure the phone for service with a VoIP provider of your choice (starting with a clean slate).