The End to End Call Simulator Test will send out ICMP echo (ping) packets. Many/most devices are programmed to take the inbound packet and simply swap the source & destination IP address and then re-queue the packet to be sent back, thus preserving the DSCP field.
Some devices will destroy the inbound packet and create a brand-new packet for the return trip. In this case, the DSCP is not returned by the end device. In this case, I would recommend testing to a different device in the environment (test to a gateway, router, or switch and see if it responds with DSCP), or setting up a remote call simulator agent to run in RTP Receiver mode and then have your local call simulator agent run in RTP Transmitter mode. That will have actual RTP packets be sent around the network, but you have to set up a remote agent in that case.
Run the call simulator in DSCP Loss Test mode to determine where the tags are being stripped.