I found this deal http://www.localphone.com/call/nigeria to call Nigeria for 3.5p per minute. It's not a calling card you can just call straight from your mobile. I'm studying in England and signed up to Localphone today, I called my mum back home and it works! You also get a free 5 minute call when you sign up so you can try it before you pay. This offer is only until 8th February, but after that it will cost 4.4p per minute which is still the best price I have seen.