Oppo
Change Default Messaging App auf Oppo
On Coloros, incoming texts land in whichever app Android has marked as the default SMS handler. Facebook's Messenger, Google Messages, and several other chat apps can all register as an SMS handler, but only one of them controls the inbox at a time. When more than one app requests that role, replies to text notifications can pop open the wrong client instead of the one you actually use.
The setting that decides this lives inside Oppo's app management area, not inside the messaging apps themselves. Go to Tap on Settings ➔ Tap on App management ➔ Tap on Default apps ➔ Tap on SMS app ➔ Set the Default SMS app. Pick the app you want to handle every incoming and outgoing text message.
Switching the default does not delete existing conversations. Older messages stay stored in whichever app previously handled them, so you may need to open that app once to check history. If a third app keeps reasserting itself as default after every reboot, uninstall it or revoke its SMS permission instead of repeating the switch.