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To read only one SMS you need to define the SMS index in Post/Get string.
If the you know the index of the last SMS, then you can read it using your mentioned string.
With string http://192.168.1.1/cgi-bin/sms_list?username=user1&password=user_pass you can see all the SMS and their indexes.
As written in wiki, you can only define the index number, but not request specifically for last or first SMS.