Advanced Call Forwarding is telling me "Sorry, but something went wrong. Please try again in a few minutes."

I am trying to help my Mom set up her Xfinity so it allows her old time answering machine to answer and take voice messages. When I go to Xfinity Voice, Settings, and Call Forwarding, the Advanced Call Forwarding box is there for a second, but then disappears and a message says "Sorry, but something went wrong. Please try again in a few minutes." I have read these forums and an employee will respond and ask for a direct message. It seems that I must ask for this type of help before the icon to send a direct message comes up. So, could you please help me out and authorize me to send a direct message with my information so that I can change the # of rings before Xfinity Voice Mail picks up to a number that is fewer than her answering maching is set at? Thank you for the help!