Hi,
The codes are direct from netflix and are their standard promo codes. The code itself, does not have a specific function, usually just a monetary value.
For example, when you enter the code it should apply a credit,usually about £41, which is the equivalent of 6 months for a standard single device user licence. If you have a higher tier subscription already, for more devices, then of course, this would not last for the full 6 months.
When you apply the code you should have seen it display the amount that was added. If you look in your billing section it will usually show more details.
If you do contact netflix, they deal with possibly hundreds of companies globally, so mentioning TalkTalk to them won't mean anything, you should just explain that you applied a netflix gift code and you think it is not working correctly. but they may need the code you used, so they can look further.
Karl.