Go to Settings > Apps > App Management > Show System Apps > Search for "IMS" > Clear Data & Cache. Restart.
Risks and important caveats
This process will irreversibly erase all your data, including photos, apps, and messages. Downgrade Oneplus 9 To Android 11
: Once the installation reaches 100%, tap Reboot . Your phone will restart into the initial setup for OxygenOS 11. Why People Downgrade Users often revert to Android 11 for: Stability : Fixing bugs found in later OxygenOS versions. Title: Turning Back Time: A Guide to Downgrading