Unlock procedure will work on E173Cs-1 Airtel India and E173z-1 MTN Sudan modems with these firmware versions:
21.157.31.00.1043 (Build: Apr 23 2012 12:02:27)
21.157.31.00.284 (Build: Mar 17 2012 12:15:53)
21.157.13.00.284 (Build: Feb 16 2012 11:05:38)

Download firmware update

Extract archive and launch “DriverSetup.exe“.

Click "Start".

Wizard will detect modem and ask for firmware code.

Launch DC-Unlocker client software.

Open Server tab, enter your username and password that you received after credits purchase. Click "Check login". (you can buy credits here www.dc-unlocker.com/buy)
If DC-Unlocker/Vygis/Rocker/Infinity dongle is connected, do not enter username and password, only click "Check login".

Select manufacturer: Huawei modems, model: Huawei E303, proceed to "Unlocking" tab.

Select "Generate Firmware code by IMEI".
Enter IMEI number.

Click "Do job" button.
Software will generate firmware code.

Enter generated firmware code to firmware update wizard and click "OK" button.

Wizard will start to update modem firmware. Do not disconnect modem or turn off computer – you can damage modem.

Remove SIM card from modem.

Launch DC-Unlocker client software.

Open Server tab, enter your username and password that you received after credits purchase. Click "Check login". (you can buy credits here www.dc-unlocker.com/buy)
If DC-Unlocker/Vygis/Rocker/Infinity dongle is connected, do not enter username and password, only click "Check login".

Select manufacturer: Huawei modems, model leave Auto detect (recommended).Press "Detect" button (Big magnifying glass).

Wait till DC-Unlocker fully detects modem.

Open Unlock tab, select “Read unlock code and auto enter to modem”.
Click “Do job”.

Wait till you see a message that modem is unlocked.

After you see that unlock is successful, restart modem.