How to send SMS-COMMAND?

Question

The GSM 03.40 specifies a message of type SMS-COMMAND which lets the sender perform operations at the SMSC (e.g. inquiry, delete, etc.) on a previously sent message using its associated TP-MR value.

I have a GSM 2G modem (SIMCom SIM808) that I use for test purposes. It is configured for SMS in PDU mode.

I can successfully send SMS-SUBMIT messages and process SMS-DELIVERY, SMS-SUBMIT-REPORT and SMS-STATUS-REPORT messages \o/

Now, I would like to play with the SMS-COMMAND messages. However, when sending such PDUs, the modem answers with +CMS ERROR: 304 which means "Invalid PDU mode".

My guess is that the SMSC somehow filters out these messages and tells the modem "invalid mode".. But that's just a guess and I hope it is wrong!

Here is my test scenario:

1 - Send a SMS-SUBMIT message

I send a SMS-SUBMIT message with the TP-SRR and TP-MR fields set to the proper value. As the destination device is off-line so the message is expected to be kept pending at SMSC.

PDU dump

PDU: 0031990B913316325476F80000050141

{
    .sca_format.raw = 0x00,
    .sca = "",
    .sca_len = 0,
    .type.raw = 0x31,
    .submit.mr = 0x99,
    .submit.da_format.raw = 0x91,
    .submit.da_len = 12,
    .submit.da = "+33612345678",
    .submit.pid = 0x00,
    .submit.dcs = 0x00,
    .submit.vp_mn = 30,
    .submit.vp_abs = "",
    .submit.vp_tz = 0,
    .submit.udl = 1,
    .submit.udhl = 0,
    .submit.udh = NULL,
    .submit.ud_len = 1,
    .submit.ud = "A",
}

AT command

AT+CMGS=15\r
> 0031990B913316325476F80000050141\x1a
+CMGS: 153
OK

2 - Send a SMS-COMMAND

Then I send a SMS-COMMAND message which is set to inquiry the previously sent message.

Note that an SMS-COMMAND set for deleting the previously sent message answers the same.

PDU dump

PDU: 00229A0000990B913316325476F800

{
    .sca_format.raw = 0x00,
    .sca = "",
    .sca_len = 0,
    .type.raw = 0x22,
    .command.mr = 0x9a,
    .command.pid = 0x00,
    .command.ct = 0x00,
    .command.mn = 0x99,
    .command.da_format.raw = 0x91,
    .command.da_len = 12,
    .command.da = "+33612345678",
    .command.cdl = 0,
    .command.cd_len = 0,
    .command.cd = "",
}

AT command

AT+CMGS=14\r
> 00229A0000990B913316325476F800\x1a
+CMS ERROR: 304

3 - Bring destination mobile online

Finally I bring the destination device back online and it receives the pending SMS-SUBMIT message. A few seconds after, I receive a SMS-STATUS-REPORT which confirms the successful delivery.

4 - Questions

Q1: Is the SMS-COMMAND message malformed?

Q2: Is the SMS-COMMAND type even supported on the SMSC side?

Q3: Any clue to make this work?