This has traditionally been the conventional way of communicating between the Kart Driver and Pit / Engineer.
The Kart is fitted with a two-way radio (Walkie Talkie) and so is the Pit / Engineer.
The Driver has a headset inside their helmet with a microphone and either two slim speakers, or a pair of in-ear noise cancelling micro-speaker earplugs, linked to the radio and PTT (transmit) button on the steering wheel with a wiring harness.
The Pit / Engineer wears a headset “Cans” with speakers and a boom microphone to eliminate external noise.
The Driver presses a button to transmit and the Pit / Engineer presses a button to reply.
They cannot both talk at the same time.
There are two types of system: Two-way Radio (Walkie Talkies) OR Bluetooth.