I was going to suggest Telestunt (
www.telestunt.co.uk : 0844 428 87 87) but they charge 4p to Nigeria. Their prices change quite often though.
I use them to call home to South Africa, and that costs 2ppm (compared to Onetel 12p and BT 75p ish [ don't want to think about that one])
I'll take a look at DialAround though.
The Grem