To streamline the classic Hangouts, Google will remove external text messaging functionality in the classic Hangouts app for Android.
Android Messages is the primary place to access SMS and Google is working with carriers and device manufacturers to include Android Messages natively in Android devices. If you are currently using Hangouts to send and receive external text messages on your Android phone, in the next few weeks you’ll see a notification in the classic Hangouts app to switch to another SMS app already on your phone or to download Android Messages if one isn’t available. This will not impact your text message history and all your messages will be accessible in whichever new app you choose.
Note: This change does not impact Google Voice or Project Fi.