Connect mobile phone to motorhome?

Hi guys. 
I have the roller team 740 Pegaso and I can hear the caller on the other side but they can’t gear me. I’ve had this problem from new 3 years ago. Dealer has done nothing to help. I’ve tried everything and it seems the built in mic is off of deactivated. Any ideas please. 

google the manual for the model of hifi/head unit, it might be in there. ive not heard of mic deactivation but that doesnt mean it cant happen.

good luck.