Allow Call Queue agents to reject calls
When all agents in a PSTN call queue have explicitly declined/rejected a call, all agents are "redialled" and receive the incoming call for a second time. The caller remains on hold during this time.
This is not ideal behaviour. If all agents have declined a call, we want to be able to perform an action (e.g. redirect to a recorded message and disconnect the call, or redirect to voicemail, or redirect to another call queue).
To be clear, if all agents are busy on existing calls, we want the caller to continue to be on hold until an agent is available.
The issue we have here is only when all agents have declined/rejected the incoming caller.