If you experience the following problem:
a) messages are sent to GSM operator, but they are rejected by the operator (they do not get through to mobile phone)
b) Modem log (menu Settings > Syslog > Gammu log) says:
Mon 2014/06/02 17:45:20 gammu-smsd: Message: testing sms gateway
Mon 2014/06/02 17:45:20 gammu-smsd: New message to send: 127
Mon 2014/06/02 17:45:48 gammu-smsd: SMS sent on device: "/dev/ttyS1" status=515, reference=-1
Mon 2014/06/02 17:45:48 gammu-smsd: Error getting send status of message: Unknown error. (UNKNOWN)
When sending SMS messages modem reports status=515.
This error almost always is due to NO or EXPIRED credit in a SIM card. Please verify if your SIM card allows to send SMS messages.