Please can anyone help. I'm trying to setup 3CX with Siemens Gigaset A580 IP Phones. I can ring via 3CX My phone app on my PC the handsets, incoming calls via my GrandStream GX4104 ring the handsets, but if I try and ring another ext from the Handsets I get "Busy". The same happens if from the handsets I try ringing an external number via the GX4104. I have turned on VOIP Status on the handsets and I get "Status 403". My 3CX is installed on a Windows 2008 Server, can anyone help?