Error message when trying to use computer to computer : Voice chat ended or session closed


I use spark 2.5.8 and openfire 3.6

All is ok for voice chat, file transfer…

But impossible to make call computer to computer, we allways have error messages :

Session closed due to Closed remotely

Voice chat ended : no media received. This may be caused by firewall configuration problems

First error message is from the one who initiate thye communication.

Before we used vpn between our offices and it worked but now all is directly from internet

Could you please explain me which ports to open to our firewall or what i can do

Http binding is activated to the default port

Media proxy is activated to range 12000 - 14000

Session Idle Timeout is 150

I join a smack debug window from a test on 2 computers on the same lan

Thank you for your help