We have two lines hooked up and fully working on the phone at this time. We have now physically connected a third line to the trunk box (and all 3 are in a hunt group) - dialing the number for this line (or having a hunt group call pop over to there) does cause the 3rd button to blink when a call is placed to it, and I can answer a call there, however the phone does not ring. How can we make the phone ring when that line is called?