SMS is stored via AT+CMGR modem command .
Before issuing this command , you have to know the SMS memory position in R520 . Memory position could
be got if at reception of SMS you acctivated the new SMS notification . If it is the case you will get +CMTI unsolicited response from modem where position has set .
The other way to get SMS is to read all of the stored SMS's via AT+CMGL command , which in turn will provide you all SMS, following by +CMGL solicited response and PDU part . +CMGL response will give you type of PDU , its status (read, unread ...) meemory position .
Please ntoe you have to write PDU decoding , as there is no text modem display for SMS in ericsson models .
If you need the AT command guideline for Ericsson R520 I can upload , but I suppose you can get via searh from internet or ereicsson's mobility world site.