You can use this method to get data out of SAP, only if the function module returns it as a parameter.
For eg. BAPI_USER_GET_DETAIL has some return parameters, which will give you back user details. But, RFC_CALL_TRANSACTION isn't for meant for returning any data from SAP.
We may be able to better assist you if you can tell the Tcode, which you are trying to access.
Thanks,
Juwin