I have a customer with an xn120 system, when the 2 receptionists are on a call they cant hear a third call. I can't find call waiting anywhere. do I have to set up a virtual extension and a button for this?

How are these calls arriving at these receptionists. Trunk termination keys or DID to the stations ext number??

all on Trunk keys, they ring to group, the 2 phones ring ok, but when those two extensions are on a call the third co light flashes, but no sound

I would hazard a guess that PRG 15.02.12 is set to "No ringing" If it is set it to "Mute Ringing"

you can try turning on second call (PRG 20-09-01).
maybe that helps.