Voice distortion on Cisco 7912 IP Phone using Free PBX on centos oS

dear guys,

I am using free pbx version Version: installed on centos os on rack mount server. I am using SIP as voice protocol and registering Cisco 7912, 7960 and SPA502G IP phone. from last 3-4 weeks i am observing voice distortion in the calls but only on cisco 7912 phone.

to resolve this i separate the voice and data on separate vlans using cisco voice vlan. even after configuring the voice vlan, distortion in the voice is still there.

one fishy thing is also observed that, while ip phone connected in standalone mode and in idle mode (no calls) average traffic of 1.2 mbps is observed on the switch port on which the ip phone is connected.