dear guys,

I am using free pbx version Version:2.8.1.5 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.

I have change the ethernet cable between core switch and IP PBX. 

below is the config on my access switch where IP phones are connected

interface FastEthernet0/13
 switchport access vlan 201
 switchport mode access
 switchport voice vlan 301
 spanning-tree portfast
!
interface FastEthernet0/14
 switchport access vlan 201
 switchport mode access
 switchport voice vlan 301
 power inline delay shutdown 20 initial 30
 spanning-tree portfast

STG traffic while no voice call and with voice call is also attached.