As zte, micromax is the famous brand for modem, huawei is also a leading brand. Many sim / network providers is using huawei modem for his network. After idea, vodafone, aircel now Bharti airtel has also come with E1731 with customized firmware and its firmware version number is and software version is Fakiro . When you will open the cover and see then there E173Bu-1 will be written and it comes with customized firmware. But, the problem is that it is locked and can not be used with other network provider sim. I will guide you to unlock your new Airtel E1731 with the help of the firmware update.
Step by step guide to unlock  HUAWEI AIRTEL E1731 E173bu-1 CUSTOMIZED FIRMWARE TOTALLY FREE :

  1. Download Universal Master Code Calculator software and firmware upgrade of E1731 (E173Bu-1) modem dongle from the download link which is provided at the end of the article.
  2. Now run Universal master code tool and generate unlock code and flash code of your IMEI.
  3. Now plug your device to your PC / laptop / Desktop and let it launch the software of modem.
  4. Now close the running software / mobile partner of the software and run firmware update E173Update_11.
  5. Now its will find your modem automatically.
  6. Click on next and when it ask you for password put flash code as your password which you had generated earlier.
  7. After a few seconds your modem will be updated with unlocked firmware.
Now update the modem with latest Huawei Mobile Partner Software from this link.

Now after fully updated your dongle, unplug and plug your modem with another than default sim (other than Airtel sim) in your computer / laptop. Now your dongle / modem will ask you unlock code / password. Put the unlock code as password which you had generated earlier and your  Airtel E1731 (E173Bu-1) Huawei Customised Firmware Modem Dongle is unlocked forever. 
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Update : If above mentioned firmware does not ask for password or unlock code the you download this firmware (Firmware Version is E1731Update_11. It will work 100% for you.