Blocking Probability

I am a bit confused about the correct way to compute blocking probability.
I have heard people use
B, prob. of congestion=(A^N/N!)/(1+(A^1/1!)+(A^2/2!)+…….+(A^N/N!)) where A=Carried Traffc and N =no of cicuits

and also
B=total overflow(no of blocked calls)/total call attempts.

Which is right for what scenario ??

Thanks
Deji