We are using Verizon as our SIP provider on a private MPLS network. We have to certify Asterisk/Elastix with Verizon to do that we have to run several tests. One of the test is to have an inbound route going to a phone that is not registered (like the phone is unplugged etc.) make a call to that number from outside and use WireShark to capture the call. When phone is not registered and I call that number Asterisk returns a 503 system unavailable message back to the provider (Verizon). Verizon is telling me that the 503 message is not correct and that is should be sending a 40x code. They say the 503 code is a code for the entire system being down. Have any of you ran into this problem or have a way I can change the error code Asterisk sends?