A call center receives an average of $\lambda$ independent calls per minute.

How likely to receive $k$ calls during any minute?

$$
p(k) =
\begin{cases}
\frac{\lambda^k e^{-\lambda}}{k!}, & \text{if $k = 0, 1, \cdots$,} \\
0, & \text{otherwise.}
\end{cases}
\\[16pt]
\text{where $\lambda$ is the average event rate.}
$$