Connecting to SIP Provider

[StardataMitel]

I'm having a great deal of trouble connecting to a SIP provider, not sure if it is something I'm doing incorrectly on the 3300 or if it is the customer's network setup.

Mitel 3300 CX. v9
Have set up Network Element for SIP provider, and Network Element for an Outbound Proxy (the customers gateway router). The provider registers based on the customers external gateway IP address, I can get Inbound calls to work okay but continue to get an error on outbound calls. It seems the packet the SIP ISP is receiving still has the 3300 Internal IP address as denoted in System IP Config. I'm being told by our Mitel Tech Support and by the SIP provider that it might be a NAT issue, but I'm not sure where to go from here.
When the Outbound Gateway has NO IP ADDRESS or has the customer's GATEWAY EXTERNAL IP ADDRESS - Inbound calls okay. Outbound calls display ERROR on screen.
When Outbound Gateway has GATEWAY INTERNAL IP ADDRESS - Inbound calls don't get through. Outbound calls try to authenticate with Provider, but fail due to packet having the 3300 System IP Address in the From field.

I hope I've explained this sufficiently for someone to help direct me, its beginning to do my head in a bit.

Thanks in advance.

 

[blackberry1980]

What type of firewall do you have,if any ?

 

[StardataMitel]

Not sure about a firewall, other than what is possibly in the DLink ADSL modem I would guess. There was a Windows Server set up as the gateway but that network config has since been changed.

 

[blackberry1980]

What type of router is doing the natting ? In most cases, routers use SIP ALG which is ment to help SIP traffic pass through NAT but causes more problems than anything else. Everytime you try to do an outbound call, a software log should be generated, can you post what this says ?
Try turning off SIP ALG on your router and you could also not specify a proxy.

 

[StardataMitel]

Its a DLink DSLG604T ADSL2 modem doing the NAT work - supposedly... All it has is a tick box saying "NAT enabled". I've tried not specifying a Proxy but then Outbound calls just give me an Error reading on the display of the phone making the call. I'll post a log of a call next time I'm able.
Thanks, always appreciate the help this forum provides. =)