I'm using a Granstream 4104 gateway (very happy with how it works). When on the PSTN line and another call comes in I hear the call waiting tone. Is there a flashook command that can be issued from the VOIP phone? (This would be useful as well as there is call conferencing on this POTS line.) Using Aastra 480i phones. (I did this before when I was using Trixbox).