Oh, I figured it out. The page is oddly designed. It presents you with a login screen at the same time as the choice of "Fusion phone numbers", without any "sign in" button in sight.
Then it waits for you to input Sonic id and password, and click on another editbox on the page. Then it suddenly checks your credentials.
"Fiber is on its way!
As a Sonic customer, you're already on the list to switch to Fusion Gigabit Fiber. We'll contact you as soon as we’re ready to schedule your installation. Thank you for joining Sonic's Internet Revolution!"
I guess it works...
Then it waits for you to input Sonic id and password, and click on another editbox on the page. Then it suddenly checks your credentials.
"Fiber is on its way!
As a Sonic customer, you're already on the list to switch to Fusion Gigabit Fiber. We'll contact you as soon as we’re ready to schedule your installation. Thank you for joining Sonic's Internet Revolution!"
I guess it works...