I use Firefox.
I cleared the cache and cookies. There were no saved passwords in Security - Options - Saved Passwords. I logged out of GoPhone account management (https://www.paygonline.com), then closed and reopened Firefox and returned to the GoPhone account management site. My 10-digit wireless number and password (displayed as four stars, as usual) were still there. I just hit the blue login button and logged in.
This is very odd because it seems to me that clearing the cache and cookies should remove that information from the form. It does not in this case.