I have an NEC SV9100 that I have set up totally as VOIP. I have 8 statiions and 8 SIP trunks. I can call out on the SIP trunks no problem. But incoming calls do not ring through unless I first use the system to make an outgoing call. To further explain, when you first arrive in the morning after the system has not been use overnight, callers will hear that "your call did not go through..." If you now make an outgoing call, you can then start receiving incoming calls. After not too long you will once again not be able to receive incoming call. The SIP trunk service is Edge Communications which is the NEC SIP trunk provider. Not sure if there is some kind of "Keep Alive" feature I need to turn on or maybe something else.