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Nomorobo is not activated automatically, you have to sign up for it.
Go here to do so:
https://www.nomorobo.com/signup
Select Comcast Xfinity as your landline carrier.
Also, you can read this for more info, but I think some of the instructions are out of date, at least they were for me when I signed up a couple of years ago (especially step 5):
https://www.xfinity.com/support/articles/nomorobo
Good luck!