docs: void handleAgentCalledEvent(AgentCalledEvent event)
      {
          AsteriskAgentImpl agent = getAgentByAgentId(event.getAgentCalled());
          if (agent == null)
          {
              logger.error("Ignored AgentCalledEvent for unknown agent " + event.getAgentCalled());
              return;
          }
          updateRingingAgents(event.getChannelCalling(), agent);
          updateAgentState(agent, AgentState.AGENT_RINGING);
      }