SIMCom SIM808 SD card problem


I am working with SIMCom’s SIM808 2G+GPS modem and trying to connect it to an SD card. I am following SIMCom hardware design docs, to save some data in SD memory.

The problem is: when I send command “AT+SD2PCM=?” and related commands for example AT+SD2PCM? , AT+SD2PCM=1 and Etc to the module, it returns ERROR message. Can anyone help me to solve this problem? Thanks.