I am fairly new to PBX system. I have recently set up a UC320W with an external SIP trunk. It seems to be registring with the SIP provider correctly and I am able to call in with my cell phone and it gets routed correctly to one of 3 SPA504G phones I have on the system. However muy problem is that I can't seem to be able to call out. The phones should be configured so that I press 9 to get a line out but I get a busy signal immediately as soon as I press any key. Any ideas how to fix this? TIA.