Cant register on at&t IP Flex

I’m using a new Virtual Didium Switchvox installed on Vmware. My SIP provider is AT&T Using IP Flex. AT&T tells me the only authentication is through the IP Address they provide. my current system is a Barracuda Cudatel witch has been discontinued. The Cudatel system has two nick cards one for Lan and the other for wan. no need for NAT.
I have the Didium setup, I can make Ext to Ext calls and I can make inbound calls without any problems. Under current sip status, it shows rejected. when I test the connection I get ISP cant ping, AT&T told me they don’t have ping turned on for the SIP IP’s. I’m also getting no response from the provider on the same screen. I have had two test sessions with AT&T and Didium support. AT&T tech is telling me & Didium tech that they are not getting any response back. The Didium Tech is telling me and AT&T there not seeing any request. so how am I able to receive an inbound call just fine if neither sees the other?
Didium setup: in the external IP I have the IP that AT&T provided. The Gateway is the Lan side of the firewall the AT&T router is plugged into. The firewall is only inline to do NAT. In the SIP provider section, I have the SIP IP from AT&T. The nat’s forward the SIP Services from the Digium system to the AT&T router. “inbound call with audit work just fine”. When I try an outbound call I receive a 503 error on my softphone “no provider”.
How do I get the Didium system to register on the AT&T IP Flex system?
Is there a way to configure the Didium to disregard the Rejected error and just send the SIP packets?
I can provide Packet Captures from the Didium.
Sorry, the Wan side of the firewall has been configured with the same IP as the External IP placed in the Digium system. AT&T said they have to see this IP to accept SIP traffic.
Sorry to be so long but the setup seemed so simple at first. Take traffic from the provider IP, NAT the packets to the LAN Address of the Digium then reverse for outbound. any thought would be appreciated.

Digium does not provide support on this forum for Switchvox, If you need assistance with that product you will need to contact them elsewhere.

Thanks for getting back with me. I have had two support call with digium and at&t on the line at the same time and both say it my problem.
I will look keep looking.