A subchannel chip was a piece of telecommunications technology designed to receive a specific signal which could not be traced.
Day 3 Edit
A subchannel chip was placed inside the phone that Stephen Saunders delivered to President David Palmer through a dead drop. Secret Service agent Aaron Pierce had several colleagues examine the phone to make sure it was clean. The built in chip was discovered, and Pierce informed Palmer that any attempts to trace Saunders' call would be futile. ("Day 3: 5:00am-6:00am")