Not sure whether we are on the same page here. The point 'pbxnsip' was trying to make is that if all agents are busy, then the caller will wait hearing the music (as if they called Dell, Microsoft and other companies support lines). But let's say, one of the agents is free and PBX sends call to that agent, agent's phone starts ringing but the agent never picks up for 2-3 minutes. Then the phone sends back some error message. In this case, PBX cannot differentiate whether the agent rejected the call by pressing 'cancel/reject' button or the phone sent the error message after the timeout.