It turn out, that the router at the server side changes destination port of the rtp packets when they arrived from the client to the server.
For example, the client sends rtp packet to the server to port 10033, but the router send this packet to port 10000.
In this way the server doesn’t received any rtp packets from client.
There is no information why when the server does not receive rtp packets tries to send rtp packets not to external IP address of the client (which can be calculated and taken from the sip-information), but to the internal IP of the client.
The problem is solved!
Thank You very much for the support.