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8-port USB GSM modem pool refuses to send messages
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June 2, 2020
11:39 am
Henk Helmantel
Forum Posts: 1502
Member Since:
May 28, 2013
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CMS error 21 and 28 are error messages from the mobile network. It indicates that the mobile network actively blocks the messages, it is not an indication of a technical problem of the modem or between the modem and the network.

Usually this error is used when the mobile network does not know where to send the message, for instance when the destination number format is incorrect, but it sometimes uses these error codes for other reasons, like insufficient SMS credits or because of spam detection. Only the mobile operator can tell for sure why the messages are rejected.

Diafaan SMS Server reboots the modem after any send error to make sure that possible modem- or network issues are resolved, the retries and reboots are a normal effect of the CMS errors. This can be disabled with the 'RebootAfterSendError' property in the advanced settings of the GSM Modem Gateway.

Regards, Henk

June 1, 2020
5:00 pm

Sadly the same happens with valid local numbers.
The GSM ports (3 different SIM cards) open and close several times - and in the end (after several retries, reboots and +2 minutes) they terminate sending the messages (3 in total) with CMS error 21 or 28.
This never happened in the past.

June 1, 2020
4:04 pm
Henk Helmantel
Forum Posts: 1502
Member Since:
May 28, 2013
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Hello Wiebe,

The most likely explanation for these errors is that the mobile network does not accept the international destination numbers of the messages, the SIM cards that you use may only allow national or regional numbers.

Another possibility may be that the mobile network does not accept the international number format of the numbers, instead of sending a message to number '+316xxxxxxxx' you could try to send the message to number '#316xxxxxxxx'. The '#' instructs the modem (erroneously) to label this number as national number.

Regards, Henk

May 30, 2020
12:46 am
Wiebe : Webwolf

hi there,

I have a problem with the 8-port USB GSM modem Wavecom Q24Plus.
It is receiving messages, but it refuses to send them.

When you send a message - directly through the GSM-gateway - one of the three gateways refuses with:
The GSM Modem returned +CMS ERROR: 21 (Short message transfer rejected) after the 0791429369014061313B0B911316311683F80000AA08E43019447E97D3 command.

The other two GSM-gateways refuse with the error like:
The GSM Modem returned +CMS ERROR: 28 (Unidentified subscriber) after the 069142831130F031CB0B911316311683F80000AA04E477390D command.

It is strange because the modem has been working for several years without any problems.
I had to do a fresh install on a new Windows 10 computer, because the old one was stolen.

Any help much appreciated.

Kind regards,


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