I’m seeing what looks like a bug in Android with apps that support sms notifications (both my own and stock apps), where if you’ve entered a number previously, you can’t get rid of it.
You can backspace over it and save, but you still get texts, and if you open the settings again your number is still there. You have to uninstall / reinstall the app to get rid of it.
Submitting bugs like this to support is the best way to do it as they get added to our bug tracker. We have talked about opening that system to the public but we are not ready to do that at this time.
I wouldn’t open it to the public if I were you. There will be an avalanche of requests that you won’t have enough resources to look at and people will become annoyed. Submitting issues through a support person is the best way to go.