you need an intercom path (key) for every person that is in a call with the switchboard (or any phone for that matter). So for instance, you want to hold a 3 party conference - you would need 2 intercom keys - for the other 3 parties in your conference. With both of those 'filled' with a call, another call to that station would get busy.
So in your example, you have one person talking to the switchboard - gets placed on hold or parked. She answers another incoming call - thats 2 speechpaths, no room to bring a 'destination' into the fray. Add another intercom key or 2 to the switchboard.
One of the reasons why there can be up to 8 intercom keys on a phone (depending on KSU and s/w)