If a carrier settings update is available, attempt to install it because it will most likely solve the problem. Installing the update may be all that is required for your iPhone to cease displaying this message. So go ahead and do the following:

• From the Home screen, choose Settings.
• Select General.
• To learn more, go to About. If you receive a message prompt that reads Carrier Settings Update after 30 seconds, it signifies an update is available. Otherwise, there is no need to install any updates.
• Tap Update to apply the carrier settings change.
• Wait for the carrier settings update installation to complete before rebooting your iPhone 6s Plus.