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Venus VT-12 CDMA USB is probably the cheapest CDMA USB modem. Below the technical specifications:

• CDMA 20001x RTT
• 800 Mhz, support Flexi, FREN, StarOne & Esia.
• R-UIM Card
• Sensitifity: -104 dBm (FER <= 0,5%) dan (-25dBm FER
• USB 2.0 compatible with USB 1.0
• Uplink & downlink speed 153.6 Kbps

Steps to install:

• Login to your box via SSH as root

• Put the Venus VT-12 CDMA USB modem to any free USB port.

• Check if CentOS recognize the modem.

  tail /var/log/messages
  

• You'll see something like these:

  Mar  8 10:24:09 centos1 kernel: usb 2-1: new full speed USB device using uhci_hcd and address 5
  Mar  8 10:24:09 centos1 kernel: usb 2-1: configuration #1 chosen from 1 choice
  Mar  8 10:24:09 centos1 kernel: option 2-1:1.0: GSM modem (1-port) converter detected
  Mar  8 10:24:09 centos1 kernel: usb 2-1: GSM modem (1-port) converter now attached to ttyUSB0
  Mar  8 10:24:09 centos1 kernel: option 2-1:1.1: GSM modem (1-port) converter detected
  Mar  8 10:24:09 centos1 kernel: usb 2-1: GSM modem (1-port) converter now attached to ttyUSB1
  

  
  Means CentOS recognized the modem.

• Create the modem configuration in /etc/wvdial.conf

  wvdialconf /etc/wvdial.conf
  

  You'll see something like these:
  

  Scanning your serial ports for a modem.
  
  ttyS0<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyS0<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyS0<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  ttyS1<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyS1<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyS1<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  Port Scan<*1>: S2   S3
  WvModem<*1>: Cannot get information for serial port.
  ttyUSB0<*1>: ATQ0 V1 E1 -- OK
  ttyUSB0<*1>: ATQ0 V1 E1 Z -- OK
  ttyUSB0<*1>: ATQ0 V1 E1 S0=0 -- OK
  ttyUSB0<*1>: ATQ0 V1 E1 S0=0 &C1 -- OK
  ttyUSB0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 -- OK
  ttyUSB0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
  ttyUSB0<*1>: Modem Identifier: ATI -- ERROR
  ttyUSB0<*1>: Speed 4800: AT -- OK
  ttyUSB0<*1>: Speed 9600: AT -- OK
  ttyUSB0<*1>: Speed 19200: AT -- OK
  ttyUSB0<*1>: Speed 38400: AT -- OK
  ttyUSB0<*1>: Speed 57600: AT -- OK
  ttyUSB0<*1>: Speed 115200: AT -- OK
  ttyUSB0<*1>: Speed 230400: AT -- OK
  ttyUSB0<*1>: Speed 460800: AT -- OK
  ttyUSB0<*1>: Max speed is 460800; that should be safe.
  ttyUSB0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
  WvModem<*1>: Cannot get information for serial port.
  ttyUSB1<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyUSB1<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyUSB1<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  
  Found a modem on /dev/ttyUSB0.
  Modem configuration written to /etc/wvdial.conf.
  ttyUSB0<Info>: Speed 460800; init "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0"
  

• Done.

Good luck!

Update:
To use these modem with TelkomFlexi FlexiNet Unlimited see the following:
How to configure Venus VT-12 CDMA USB modem with TelkomFlexi