How to force NAT not to change outgoing ports?

Question

SIP

Available:
Windows Server 2012 R2 Standard, standard NAT is raised.
Several 3CX soft sip clients.
The following situation occurs:
The client sends RTP packets to the ports specified in its settings.
NAT forwards them from arbitrary ports. But port changes occur within the same connection.
For example: port 63308 is indicated in the invite, the first RTP packet comes from port 63388, the sip server adjusts to the changed port, but the second and subsequent RTP packets again come from us from port 63308. At the same time, they always come to nat from LAN from the same port .
In the picture there is a packet dump, on the left - that came to the local interface from 3CX, and on the right - sent by NAT from the external one.

The provider does not adjust for the second time, continues to send RTP traffic to port 63388, as a result, one of the parties to the telephone conversation is not heard.
The problem manifests itself on two different physical machines with different OS versions - 2010r2 and 2008r2.

Answer 1

You are confusing something with outgoing ports and destination ports.
If there is no re-INVITE, the client MUST NOT change the outgoing port, nor the destination port.