Connection made be simple or wvdial for ppp connections
Small test for UMTS dongle (huawei e352b, seriously, Huawei :D) shows that wvdial is a pretty good for quick connections via 3G modems. Also, it has a lot of useful options.
So, before made connection we need to install usb-modeswitch (usb_modeswitch in gentoo) and (in some distros - usb-modeswitch-data).
It has udev rules and binary file, all this will switch usb dongle from usb-storage (or something else?) into usb-modem mode.
Next, run wvdialconf to generate config file. wvdialconf will send commands to dongle and try to write optimal parameters in /etc/wvdial.conf.
Sure, if you know correct options for modem - just write it to /etc/wvdial.conf.
Below correct config file for Utel (ukrainian operator)
[Dialer Defaults] Init1 = ATZ Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 Init3 = AT+CGDCONT=1, "IP","3g.utel.ua" Modem Type = USB Modem Baud = 460800 New PPPD = yes Modem = /dev/modem ISDN = 0 Phone = *99# Username = 1 Password = 1 Stupid Mode = 1 Auto Reconnect = on
Also, maybe /dev/modem doesn't exist. Then create symlink from /dev/ttyUSB0, or set /dev/ttyUSB0 in wvdial.conf.