Cisco 7200 NAT server, release 12.4(10) FC1. Problem: SIP messages content with the NAT address also changed?

Question

My ISP added a new NAT server to his network, because we're using local IP addresses.
The problem is that the NAT server that they are using is also modifying the content of the SDP portion of SIP messages and change the IP address there, whenever they find the sequence that represents the public IP address and put the local address instead.

They are not doing any port manipulation. just basic allocation and translation of the public IP to the local IP.

Answer 1

Are they doing port address translation?

From a document I found:
If an endpoint supports Connection Oriented Media, then the problem of symmetric NAT traversal
is solved. Two scenarios are still problematic:
1. If the endpoint does not support the a=direction:active tag.
2. If both endpoints are behind Symmetric NATs
In either of these cases, one solution is to have an RTP Relay in the middle of the RTP fl ow
between endpoints. The RTP Relay acts as the second endpoint to each of the actual endpoints
that are attempting to communicate with each other. Typically, there would be a server in the middle of the SIP fl ow (herein called a NAT Proxy) that would manipulate the SDP in such a way
as to instruct the endpoints to send RTP to the Relay instead of directly to each other. The Relay
would set up its own internal mapping of a session, noting the source IP:port of each endpoint sending it RTP packets. It then uses that mapping to forward the RTP from endpoint to endpoint.