If you decide not to use an Ip Address with any sip device on your sip user account and someone gets access to your sip password they can make calls using your account. Ip Address locks your sip account to an ISP (Internet Service Provider) network and device, making your account that much more secure. Ip Address works best with static ip addresses, but can be used with dynamic ip addresses.
To view your device and Ip Address, sign in and from your dashbaord click on Account from the menu to view your active registration, click on Device to view Ip Address (Network).
To update your sip user account go to your Sip page and add the IP Address to your allowed IP address range.