In a Tweet posted by @Android today, Google and AT&T have agreed to make Google Messages the default messaging app for all Android phones sold by the carrier. While most Android devices already come pre-installed with Google Messages, Samsung phones continued arriving with Samsung's built-in messaging app that did support RCS, but does not support Google's recently enabled end-to-end encryption for one-on-one chats.
With T-Mobile agreeing to do the same a few months ago, and now AT&T pledging to completely switch to Google Messages' RCS, Verizon is the only major US carrier left to make the pledge to make Google Messages the default app for messaging on its Android phones. As of now, Verizon continues to pre-load its self-branded Messages+ app as the default.