No, No
This is not a call waiting problem.
I have IP403 with 4xBRI
Internal calls are OK, external calls are wrong.
When I call (from external network for ex.: mobile phone) I don't have bust tone. On call status I dont see any waiting calls (when I turn on call waiting then I see waiting calls). I see on SysMonitor that this call is incoming. I think that is problem with NT configuration.