A customer is using a Yealink T32 with Plantronics headsets. When establishing a call inbound or outbound using hanset the call is crystal clear. When using the Plantronics headsets there is a very slight hum in the background of the call. The HW261N headset is very slightly better however the hum is still present. Have played with CODECS however the hum is still very much the same. Any suggestions would be greatfully received.