When you call the main number after 4 rings the voicemail grabs the call and without a greeting hangs up. I had the customer put a button on their phone for ext 19 and 20 which are the voicemail ports and you can see them light up after the 4 rings. I also had them plug ext 18 into trunk port 4 and call ext 18. When you do this the voicemail says ext 10 is unavailable. What gives?