It can be seen as a bug, because 's' is not handled in the code. Thanks for reporting this.
While waiting for the next version, use the header To_TOA: National
and give the number without the s prefix.
Or, here is the fix, if you want to apply it to the code and keep the message as it is. File smsd.c:
writelogfile(LOG_INFO, 0, "I have to make a voice call to %s, with (%i times) DTMF %s",
// 3.1.15: Fix: Did not make a voicecall to short number starting with 's'.
if (*to == 's')
sprintf(command, "ATD%s;\r", to + 1);
if (set_numberformat(NULL, to, to_type) == NF_INTERNATIONAL)
sprintf(command, "ATD+%s;\r", to);
sprintf(command, "ATD%s;\r", to);
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