Mean busy period of an M-M-infinity queue
= Mean busy period of an M-M-infinity queue
Writing $\rho=\lambda/\mu$, the stationary empty probability is $e^{-\rho}$. Alternating idle periods of mean $1/\lambda$ with busy periods and applying the renewal-reward theorem gives
$$
\mathbb EB=\frac{e^\rho-1}{\lambda}.
$$