Claude is right, after re-arranging the screws so that the top two are the longest, bottom-right shortest and bottom left is the second-shortest the in-call audio came back with Bluetooth calls.
The same was found and posted by Pete on the Apple Support Communities forums and, as mentioned, I can also confirm that this fix works.
Hope this helps.
Unfortunately I forgot to check at dis-assembly which screw goes back where at step 11 so I ended up following the instructions although, I would find it logical that the longer screws are for the top and the shorter ones for the bottom holes, which is what you mentioned also . I anyway ended up following the instructions and all went well except that I am now unable to hear anything while in a phone call connected to the Bluetooth in my car. The phone connects, I have the battery and network status on the screen of my car, I can receive SMS on the car's screen, I can stream music to the speakers but I cannot hear anything while in a call. I can hear the call ringing in the car speakers, I can answer it but after that it's silent.
So, I was wondering:
1. Did you hear back on your comment from iFixit?
2. Did you install the screws back the way you mentioned in your comment? Did it work?
3. Did you check / note down the position of the screws at dis-assembly?
Thanks in advance.