Yes they do work. I used to use a Speak and Save phone card to call USA and Canada. With these cards you can call the 0800 number but this burns the card quicker, most also come with a 0845 number which reduces the burn rate on the card but also charges the 0845 call to your NTL account.
Also, its worth checking around on these phone cards because often if you buy them in £20 values or above you can get 25% added for free.
have a look here for comparisons
http://www.1st4phonecards.com/lp/GADUK1/
Something i used to do was call the 0845 access number using my mobile. This way i got the lowest call rate on the phone card, and i got to use up my free monthly minutes on my mobile.