There are two basic rules:
1) If you get problems uninstall the version provided by your distribution and use the official git version.
2) If your phone was made recently (say year 2004 or later) use one of the following configurations:
Non-Nokia phones, using USB cable:
[global] model = AT connection = serial port = /dev/ttyACM0 # check dmesg for the right name
Non-Nokia phones, using Bluetooth:
[global] model = AT connection = bluetooth port = 00:11:22:33:44:55 # replace with the address of your phone
Nokia phones, using USB cable:
[global] model = series40 connection = dku2libusb port = 1
see gnokii wiki on how to set permissions to use this as non-root.
Non-Nokia phones, using Bluetooth:
[global] model = series40 connection = bluetooth port = 00:11:22:33:44:55 # replace with the address of your phone