Hello nib in_s,
Welcome to Apple Support Communities. I understand that you would like to stop the notification regarding iCloud, “Your carrier may charge for SMS messages used to activate iCloud. I’m happy to help.
Let’s start by restarting your iPhone using the steps here: If you get an error when trying to activate iMessage or FaceTime - Apple Support Does the notification still appear?
Are you trying to activate iMessage or FaceTime? Go to Settings > Messages > iMessage. If so, this message is provided to let you know that you may be charged for the SMS message used to activate it.
Take care.