If mute is selected (using the Plantronics USB headset's mute button or the SoftPhone GUI's mute button) while the Real Time Protocol (RTP) streams are already up between the client PC and another device, then CPU utilization of SoftPhone (the CiscoIPSoftphone process) goes from around 10 percent to 75 percent. The increase in CPU utilization causes choppiness in SoftPhone conversations.
This issue is documented in Cisco bug ID CSCdy33907 and resolved in SoftPhone Interim build 1.3(2.4) and later.
This problem has been noted only when the call is made with the G.729 codec. A workaround to this problem is to use G.711.