Simply follow the steps to unlock the Huawei modem which is not asking for unlock code:
1. After flashing the airtel E173CS-1 or any another modem download the Huawei Modem Code Writer Tool from the link which is provided at the end of the article.
2. Extract the zip file and run the Huawei Modem Code Writer.exe.
3. Now plug your flashed Huawei modem in your computer.
4. Now click on “Please Select Com Port” button and it will open a small window.
5. Click on Detect button and select the port.
6. After selecting the Com port click on Accept button.
7. Now click on Unlock Modem button.
8. Now its will open a new window asking for unlock code.
9. Here, provide the Unlock Code (8 digits) which you have already and after that click on OK button.
Now you have an unlocked Huawei 3G modem in your hand. Enjoy!!!