Walkie talkie feature is supported with the help of the WT-100 accessory sold separately on https://www.coros.com/accessories.php. Each helmet requires a WT-100 to be connected to enable this feature. Once WT-100 is paired to the LINX, you can use the smart remote to easily control the communication.
OMNI & SafeSound
Walkie talkie feature is NOT supported on OMNI and SafeSound helmets. You can receive phone calls and talk via the wind-resistant microphone in the helmet.
The reason why there's a walkie talkie button on the smart remote is because the walkie talkie feature was removed before the product launch due to technical difficulties.