@Josele13 I have the exact same problem.
BQ E4.5, I can connect it to my Audi.
When I play music on the smartphone I can hear it in from the car's speakers.
I can even see my list of contacts in the phone form the car dashboard.
I can start a call and I can receive a call but, there is no sound. I cannot hear the other person and they cannot hear me and it's the same in both directions when I call someone or when someone calls me.

I have an Aquarius BQ 4.5.
Sometimes when I activate cellular data connection the phone finds it and displays the H symbol in front of the sim card name in the antenna setting panel (the one that you get by swiping your finger from the top of the screen). However, the top row of current settings does not appear to "turn on" the H sign near the GSM antenna as if there is no connection. Rebooting does not help and going to the flight mode and back does not help neither. It does not always happens like that, I'm guessing it somehow depends on the antenna or the provider (e.g., when I'm aboard and I activate the data roaming in the settings).

Have anyone seen something like that or had the same problem?