How do I get a second call while am on the phone? I have a Grandstream, a cisco, and a aastra phone installed on elastix 1.5.2, and although I have all the line buttons setup on the phone they seems to only be able to get one incomming call at a time, the second call goes to voicemail all though am on only one line (button). what am I doing wrong? help. Note: I program all the buttons with the same extension number.