I have fitted a Sat Nav retrofit with a large screen head unit with an odroid dispaly & functionality. Mainly it is working well
however the bluetooth system is not - it has the correct modules fitted as required on a optical loop and a microphone overhead.
I am getting the following message when selecting the phone on my keypad.

HANDSET IN USE
MUTE & REPLACE HANDSET
TO CONTINUE HANDS FREE CALL OR REPLACE TO END CALL

If I had the optical loop the wrong way round between the Satnav CD & bluetooth box could this account for it ?

any ideas ???