Most pre-2012 Huawei modems can be unlocked by IMEI for free here: http://www.mapmyplace.com/calc/hua.php
Do note that the Huawei E220 can’t be unlocked by code calculation and requires a cable/server option and this is not free.
New Huawei modems from 2012 use an encrypted algorithm and sadly can’t be unlock for free. The following site offers Huawei unlock codes for these new modems at £3.50 each which shouldn’t break the bank.
E173, E1732, E1815, E352, E353, E353Ws,E353Ws-2, E357, E362, E367, E368, E372, E392, E398, E589, E5832s and E589u-12 LTE
The site address: http://huawei.site90.com/
The site also has links to the latest version of unbranded Mobile Partner.
This thread is about Huawei MODEM Unlocking, the little plug-in dongles that people use to connect to the internet.
The list of supported Huawei MODEM devices is given by both sites listed.
I'm pretty sure that the Huawei G300 is a phone so, you need to find somewhere that unlocks phones!
P.S. Having just used Google, it seems you can buy the G300 unlock codes on E-Bay for just under £5 each!!