hey, jra,
After doing a little bit of digging, I found a post from 2024 claiming that it is not possible to read SMS messages via Modbus: Read and Send SMS via Modbus
I believe the initial reply was written by one of the engineers from Teltonika, but the account seems to be gone.
As Matas suggested, you’d be better off with using the API, or playing around with the log command while filtering out only the SMS specific logs to get a confirmation of an SMS being received/sent.