I had that on a tde600, and they did not want to have to press a second key for internal call waiting so what I did is made a icd group and called it operator, set the operator extension (under system>operator) to that group number, set call waiting distribution to all. set call waiting on the operator extension. it will bleep and show on the display.
As for external calls the call waiting as Kxtvet said will solve that as well

“I have not failed. I've just found 10,000 ways that won't work.”