The +CNMI command subscribes and configures new message
indications.v2.0.x
For reference, see 3GPP 27.005 Ch. 3.4.1.
Set command
The set command registers or unregisters an SMS client. Only one AT client can be registered as an SMS client. An existing registration must be released before registering a new client.
Read command
The read command is used to query how new messages are indicated.