sip trunk by using NAT

[Htc]

Hello, I am using OXE v11.
Recently I needed to use SIP trunk to connect to ISDN.
SIP provider gave me the SBC IP address - Default Gateway - Subnet Mask and my own IP address.
I asked the network administrator to create the NAT settings so that he could convert the IP address of CS2 to the address defined by SIP provider. These things were done and I also configured SIP trunk. Finally, when I tried to connect to ISDN, the phone ringing but after picking up the phone, no sound was transmitted. I asked the SIP provider why this happend. After checking the SIP packets, he told me that he sees the IP address of CS2 as the source device, not the address defined in the NAT. The network administrator also says that we have transferred the addresses correctly and that this is your problem. Now I want to know Is it possible to specify another address ( in a different range than the CS2 address) for the SIP trunk source? Or is there another solution (other than using a peripheral device) for me?