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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

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