Normal distribution in call center(queq theory) help?

In a call center(within a very large city) with N servers the average time between 2 call arrivals is 20s(exponential distribution) and the length of the calls is a normal distribution with average=200s and std. deviation=1s. Calculate the probability of a call being blocked P=f(N) for values of P between 0.1% and 20 %

Any clues?
If the distribution of the length of the calls were exponential also,we would have an Erlang B system,no big deal.But now im deeply confused.

Thanks for your time,
Fotis