My understanding of this bug
Is that while the phone 'rings' for the incoming Facetime call, anything said during that time can be overhead by the other participants in the Group FaceTime. It rings a limited number of times, so your window of opportunity would be limited. You'd have to keep calling back over and over again to record an entire deposition.
How does that work, even with the phone on silent it will vibrate and the screen will light up. No one noticed a phone doing that during an entire deposition? If it is in a pocket/purse/briefcase its ability to "overhear" would be limited, it would have to be out in the open to be capable of capturing everything said (even then it would probably miss stuff said by people in the opposite direction of its microphone, we all know cell phone microphones are not studio quality omnidirectional jobs)
I think this lawyer is very likely of the ambulance chaser persuasion.