Very strange behavior using Voip Application over VPN

[jeanblu]

are connecting to VPN just fine. They are using Forticlient 6.0.9. But the following behavior is occurring only in one particular VOIP application that is affecting the VPN traffic.

Users connect VPN. Tunnel is estabilished and the users access the internal company network.

After the users open the voip application, all the traffic to the company appears to be lost. But the tunnel keeps up. After exactly 5 minutes, the traffic is reestabilished, including the Voip application that works fine.

I don't see any error log on client or on the Fortigate server.

Does anyone can help me understand whats going on?