DSX tech support said the same thing, but in reality DSX does know there is a 2nd call and third and fourth and all of them come in thru one SIP trunk (one number. It is also true for outgoing calls, I'm able to make 4 simultaneous calls.
I don't have any CO lines connected to DSX.
It doesn't make any sence, but it is working
If you are curious I can let you connect to the system so you can see for yourself.