Hi,
here the answer from sierra. This workaround works properly for me!
There is a command AT+CGSMS which is used set the MO SMS transport service. On FX100/ Q2698, its default value is set to 3. Can you please confirm the response of AT+CGSMS? command and if it is not equal to 1, set it to 1 using AT+CGSMS=1. And after that try to send SMS while application is running.
We have tested it and it is working fine with AT+CGSMS=1. In other cases, we are also able to see the issue.
Test once again with AT+CGSMS value set to 1 and let us know in case of any issue.