Before revision:   On finding the server unavailable ( busy or failed), inbound calls join the orbit and reattempt for service at exponentially-distributed time intervals.
After revision:  The incoming calls arriving at the server follow a Poisson arrival process, while outgoing calls are made in an exponentially distributed time. On finding the server unavailable ( busy or failed), inbound calls join the orbit and reattempt for service at exponentially-distributed time intervals.