Hi,
i use WS228 version L23 and try send SMS.
AT&F
OK
AT+CREG?
+CREG:0,1
AT+CMGF=1
OK
AT+CSMP?
+CSMP: 17,167,0,0
OK
AT+CMGS=“+7…”
Test message — Not accept CTRL-Z but ESC accepted
How to resolve this ?
P.S. Please don’t suggest change to Telit GE865
How are you communicating with the WS228?
Are you sure that whatever you’re using can & does properly send a Ctrl+Z… 
I use rsterm terminal
Also for test i connect Telit GL865 and SMS works fine.
What is that 
And are you sure that it can & does properly send a Ctrl+Z
Yes - it sent Ctrl-Z, because Telit modem work correctly and Serial sniffer display it in log.
Ah - you mean this: easyvitools.com/rsterm/index.html
Maybe this is what’s confusing the Sierra Wireless (non-Telit) WS228…?
Have you tried using some “neutral” terminal such as hyperterminal or puTTY