NoMoRobo, one of the most popular of all, which works on internet based landline phones (VOIP), as well as iPhones and Androids.
Finally, though not exactly an app, it's worth it to make sure you are still on the "Do Not Call" list. It won't stop scammers, but will keep legitimate companies like banks and warranty companies from calling.
Booher may give a couple of them a try, so he can get his quiet lunch break back again.
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