This procedure is for a Huawei E1731 USB modem that I got from Airtel.
1. Install wvdial
  sudo apt-get install -yq usb-modeswitch usb-modeswitch-data   wvdial
2. Configure modem (Make sure you replace the<phone number> placeholder with your actual phone number.

sudo gedit /etc/wvdial.conf

[Dialer Defaults]
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
Modem Type = Analog Modem
ISDN=0
Phone = *99***1#
Username = <Phone number>
Password = <Phone number>
New PPPD = yes
Modem = /dev/ttyUSB0
Baud = 406800
Stupid Modem = 1


3. Run wvdial

sudo wvdial