Consider the following topology where some sniffer software is running on the PC:
PC with sniffer software < PC port ( IP Phone) switch port > Cat6K with VLAN configured (no aux VLAN)
When a call is active on the IP phone, the PC with the sniffer software attached sees all of the Voice over IP (VoIP) traffic. The IP phone appears to be acting more as a hub than a switch.
This is potentially working as designed, supporting dual homing. This allows possible future built-in redundant links for very high availability applications. Since voice bandwidth is incredibly small, no adverse behavior should be caused by this operation.
The 79xx series IP phones all contain a three-port switch, one of which is internal. It streams VoIP traffic on both external ports.
For more information on the operation of the 79xx series IP phone three-port switch, refer to chapter five of the Cisco Press book Troubleshooting Cisco IP Telephony [ISBN: 1-58705-075-7].