I'm not 100% sure here, as I don't do unsupported phone via SBC (I do them via VPN/LAN always), but I suspect if it doesn't show via the SBC (the the via line) then it's not using the SBC and instead it's using Direct SIP (often called STUN on the forums).
Look for the outbound proxy settings in the device. Put the IP of the SBC and port 5060 there.