Skoda Octavia ‘robotic’ sound when trying to use the hands free kit

When I make a call using the hands free all I can here is a metallic ‘robotic’ crackle, it does the same if I speak to Siri in the car. However it doesn’t do it when I stream/play music in the car. The most annoying thing is that it doesn’t happen all of the time. I have reset the radio and unpaired my phone but nether helped.