A Cisco IP phone receives untagged data frames from an attached PC. What does the phone do with that traffic?

Cisco IP phones are designed to act as a mini-switch, often with a built-in three-port switch: one port for the uplink to the network, one for the phone itself, and one for a PC. When a PC is connected to the phone, the phone typically passes the PC's untagged traffic through without modification. This is because the PC traffic is usually intended for the data VLAN, and the phone's auxiliary VLAN feature is designed to handle its own voice traffic separately. The phone tags its own voice traffic with the voice VLAN ID but leaves the PC's untagged data traffic as is, allowing it to be forwarded to the switch on the other end, which then places it into the appropriate data VLAN based on its port configuration. Dropping the traffic or tagging it with a native/default VLAN would disrupt the PC's network connectivity.