AT&T and Google announced that all Android phones will be able to use Google's Android Messages app, which provides SMS and RCS services. T-Mobile signed the same partnership agreement with Google in March. Verizon is the only US carrier that has not committed to switching to Android Messages as a default.
With the switch to messages comes another important shift: interoperability between RCS and other networks. AT&T has supported RCS since a while, but it has not been as consistent as it could have been. AT&T customers will also benefit from the rollout end-to-end encryption to RCS by Google this year. This rollout actually began earlier than expected.
RCS has been promoted by Google as its default SMS solution for Android. It is an open standard that can be adopted by any carrier as the next generation SMS. RCS offers many advantages over SMS. It can send larger files and has no character limit. It can also show typing indicators. RCS can offer group chats with better support, Wi-Fi support, as well as end-to-end encryption to allow for one-on-one conversations.
Google is to blame for this confusion.
Android Messages will detect if you're texting with another Android phone that supports RCS. Your text entry window will change to indicate that you are sending Chat and that Chat features have been enabled. This is not the same as Google Chat, which is the company's other messaging service. Google is to blame for this confusion.
Despite Google's best efforts, carriers have been slow to adopt RCS, in spite of their best efforts. They announced in October 2019 that they had abandoned their attempt to form an RCS consortium. Google had to decide to handle the situation after years of a long transition, offering RCS services directly for Android users.
The RCS Chat rollout was a disaster because of politics and corporate fights as well as confusing Google messaging app strategies.
Google is now making it a default with the carrier deals. It's just one step. Verizon and Apple will also need to join the fray. RCS is not supported by the iPhone and Apple has not yet made any announcements about it.