Most deals require a new line and or a "buy one", this works even with an upgrade, so if you have an existing plan and any of the following phones, you may as well just upgrade your phone.
Eligible devices:
Apple: iPhone 7, 7 Plus, 8, 8 Plus, X, XR, XS, XS Max, 11, 11 Pro, 11 Pro Max
Samsung: Galaxy A50, A8+, Note8, Note 9, Note 10, Note 10+, Galaxy S8, Galaxy S8 Active, Galaxy S8 Duos, Galaxy S8+, Galaxy S9, Galaxy S9+, Galaxy S9+ Duos, Galaxy S10, Galaxy S10 5G, Galaxy S10+, Galaxy S10E
Google: Pixel 2, 2XL, 3, 3 XL, 3a, 3a XL
LG: G8 ThinQ, V40 ThinQ, V50 ThinQ
Motorola: Z4
Other: RED Hydrogen One, Razer 2, OnePlus 5
https://www.att.com/buy/phones/samsung-galaxy-note20-5g-128gb-mystic-bronze.html?offerid=15500006