S60 Sat Nav/Phone: Parrot CK3100 on HU-850, no sound

[MRkSTL]

Hi,

I bought the car with the Parrot CK3100 kit already installed and didn't check it for a long while. It occurs that the mic works ok, it connects with mobile phones etc. However there is no sound from the speakers during the call.

I took the radio out in order to check the wiring ,but it all seems to be connected with no broken wires. There was just one plug and a connector loose, so I connected them together ,but that didn't help.
I haven't checked the blue box because I would have to remove the glove box and I simply didn't have time that day.

Any suggestions what could it be?

Thanks