Note: You must have the SIP firmware installed on the phone, not the SCCP or the MGCP firmware. Flashing the phone is outside the scope of this tutorial, you're welcome to try searching here.
There are two methods that you can use to configure the phone. The simplest way is to configure it using the settings menus although there are some settings that you cannot set in this way to unlock the more advanced features of the phone, for this you'll need to use a TFTP server which you can find more details for elsewhere online.
To begin, you'll have to configure some IP settings. Hit the settings button on the phone to bring up the settings menu. This button is either labelled "Settings" or looks like a box with a tick in it depending on where your phone was originally purchased.
All the settings are initially locked. You must unlock the settings by going to menu option 9. Either use the arrows to navigate to the option and use the soft key to select it, or just press 9. You'll be asked for the password. If you do not know it, the default password "cisco" may work. Enter it as if you were writing a text message using the numeric keypad and then press the "Accept" soft key.
You'll now be returned to the settings menu. You should now select option 3 for "Network Configuration". If DHCP is active on your network, the phone will have requested a DHCP lease and a lot of the information will have been filled out for you. The phone should have a static IP address whether in global address space or private. You can arrange this by having the DHCP server assign a fixed address every time the phone requests one. The phone's MAC address is displayed in this menu rather convieniently.
The important ones to configure are:
Once configured, hit the "Save" soft key. Now from the settings menu, you should select option 4. If you are behind a NAT gateway, set option 24 (NAT Enabled) to Yes and option 25 (NAT Address) to your external IP address. At this time you should also forward UDP ports 5060, 16384-32766 to the IP address of the phone.
You're now ready to configure the line settings. Option 1 in the SIP configuration menu allows you to enter your SDF VOIP details. These should be as follows:
Hit the "Save" soft key and once you return to the home screen, you'll be connected. Hit the "New Call" soft key and dial 1004 to hear the milliwatt test tone to confirm it's working.