How to remove Your carrier may charge for SMS Messages used to activate FaceTime
Source

You didn't mention the iOS version you're using so I'm assuming you're on the 11.1.2.  Here's a few things to try (not in any particular order):


Make sure that your date, time, and time zone are set correctly. Turn Messages off and on in Settings &gt; Messages. Open Message, under the Message settings, a popup message will appear saying your carrier may charge for SMS messages and select “Cancel”. Message activates itself after you have clicked Cancel.
Make sure that your iPhone has the latest version of iOS. In not, backup your iPhone and then simply update. 
Remove and reinsert the Sim card. Apple explains how you can do that.
Tap Settings &gt; Your Name (and iCloud ID). Then tap Name, Phone numbers, Email. Under Reachable At tap Edit and add your phone number.
Reset network settings. Go to Settings &gt; General &gt; Reset &gt; Reset Network Settings.  As the action states, this will reset your network settings (e.g. your Wi-Fi passwords will be removed etc).


If you are still having this problem, contact your cell carrier (AT&amp;T, T-Mobile etc.). You may also contact Apple support.