Q7521020: When I send a test SMS message using my GSM modem, I get the following error: 30103. What does it mean?
This means that the port you are attempting to open is already in use by another application. You only get this error if you use a COM port (like COM1) rather than a Windows Telephony device (like 'Standard 9600 bps Modem').
Check if there's another application that is already using the COM port. In some situations, you may want to benefit from a thing called 'port sharing', where one port can be shared by multiple applications. To benefit from 'port sharing', you must use Windows Telephony devices (like 'Standard 9600 bps Modem').