To edit your Sip User Account, go to the Numbers page.
Always use IP Addresses for all your sip accounts. This option works best with a static IP Address, you can use dynamic IP Addresses, add each IP Address on your network to your white-list of IP Addresses allowed to connect to your Sip Account.
There is no limit to the number of IP Addresses you can add to your white-list.
Go to your Numbers page and click on Sip Accounts.
See below for more details.