your telephone bundled with television and internet....when I was a cable subscriber to AT@T if my telephone was busy I would get messages like that if I was watching television.
While we do have a Comcast line it works through the computer and is not linked to the TV. . . and none of the features for phone notifications on the TV are activated. So I doubt that has anything to do with it.
I used to have that notification feature when I had DishNetwork in Stockton.