Insignia NAVI 600 phone interference at 3rd party end only

[CJP]

Hi, hoping someone might be able to advise please...

my [B]2012 Insignia 2.0 CDTI has the NAVI 600 unit. It appears to work (sometimes get sniffy with the Satnav SD card and USB) but ...
when I'm on a call via the bluetooth connection the other person gets a lot of interference, which I don't hear - my end is fine. The interference also appears to be worse the faster the engine is running.
Thanks for any advice you can give.