Connection Lost

[BenTho]

Hello I'm Ben, a young IT Technician from France.
Firstly sorry for my rusty english.

I'm working for a compagny wich use Ip Deskphone for our client.

Client send me the IPBX adress, i can reach with the soft (i can settings, use the menu) but after 1-3 minutes i have a "Connection lost" message.
I set a rule in the firewall that allowed everything on TCP and UDP ports, i disabled our antivirus too.

We absolutly need a vpn connection (behin a citrix netscaler). I tried with a computer outside our domain, a personal computer of mine with our vpn, it's worked.
I'm not in charge about network's securities for my compagny, may be a gpo?
I tried to check QOS parameters but i'm not confortable with it.

Could you give support?