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Do you allow softphones to connect directly from 1 softphone to another? As that is how your voice data normally travels with SIP. This might be required between VLAN's in your setup and not anticipated.
In some firewalls this is taken care of by listening to the SIP requests and open the ports dynamically as needed. But in some cases you need to open large UDP ranges between segments.
In you case you need to capture on 2 softphones and see what the send and receive to see if you can point to a firewall.
Don't rule out an issue with the windows firewall if you run a softphone on Windows.