The VW voice activations system is not the same as Siri. Siri works out of the box as standard, when you have your iphone connected either via Bluetooth or CarPlay and does all the usual "Hey Siri" things - "Hey Siri, Call John Smith", "Hey Siri, Message Mum" "Hey Siri, directions to Home" etc. Just press and hold the voice activation button (RHS of the steering wheel (at least it is on UK versions)), wait for the beep and say "Hey Siri....."
The VW voice activation system is a paid for option (though my United has it as standard) which uses "Hey Volkswagen" to activate functions. I've messed around with it but it doesn't actually seem to do anything useful though it seems to allow you to change radio stations, media sources on the head unit etc. Doesn't seem to do any car-related functions such as "Hey Volkswagen, switch wipers on" which would be a useful thing