Some modems expose more serial ports and you need to carefully choose the right one - for example only one of them can receive USSD notifications.
On Linux, you might have to install usb-modeswitch to make your modem acutally behave like a modem and not like a disk containing drivers for Windows.
Example configuration on Linux:
[gammu] device = /dev/ttyACM3 connection = at
Example configuration on Windows:
[gammu] device = COM12: connection = at
The only support for Symbian phones is using applet installed to phone and Bluetooth connection. You should use blues60 Connection and Bluetooth address of phone as Device. On older Symbian phones you will have to use gnapplet and bluerfgnapbus connection.
Do not forget to start the applet before trying to connect to the phone.
[gammu] device = 11:22:33:44:55:66 # Bluetooth address of your phone connection = blues60
If you have Series 40 (S40) phone, it should work using either Bluetooth or USB cable.
For USB cable choosing the right connection type is more tricky and depends on generation of your phone. Newest phones usually work with dku2 and the older ones with dlr3 as Connection.
Should you have old phone with serial cable (and USB to serial converter), fbus Connection is the right one.
Example configuration for Bluetooth:
[gammu] device = 11:22:33:44:55:66 # Bluetooth address of your phone connection = bluephonet
Example configuration for newer phones:
[gammu] connection = dku2
Example configuration for older phones on Linux:
[gammu] device = /dev/ttyACM3 connection = dlr3
Example configuration for older phones on Windows:
[gammu] device = COM12: connection = dlr3