NAT issues

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Hello, this isn’t an Asterisk specific problem but I don’t know who
else to ask for help.

This is my setup, it oftens finds double NAT situations:

[Asterisk box] <-> [Firewall IPCop] <-INTERNET-> [Random Router] <-> [Softphone]

In certain situations, when two or more client softphones use the port
5060 at the same time and try to register, the UDP translation state
of the port fails to assure the connection and drops both phones.

If I change the client ports to random ones, they register, they can
make calls and everything.

It just happens if there is port clashing. I am not sure how to tackle
this situation as enforcing random ports to the softphones is not
viable for the setup.

Is this a problem with the IPCop Firewall? I tried flushing the
conntrack tables yet this situation kept happening. It gets to the
point that nobody can use the 5060 port after a while (when everyone
is trying to register).

Thank you,

Perssy Llamosas.