Bypass Qos

Hi, i have a installation with about 40 Snom D785; some computers on infrastructure, use bypass on the phone. I split network with two vlan: one for voice and one for network. When i open a large file on pc connected to phone bypass, voice quality is poor, on most case, call drop because large network traffic is generated from pc. Any idea where i can use QoS on phone for mitigate this effect and reserve 50% of gigabit band for phone when call is in progress ?

Thanks.