Hi Javier! Thanks for reading it. Well the fake phone call was quite simple, we usually broadcast our usability tests to another room in the company so the rest of the team can watch whilst taking notes of the test (we normally use Skype for this).
During the test the user was presented a task to call the one of her contacts (there was a screen with a big call to action on a phone icon), when they pressed it the fore-mentioned native looking android caller would pop (everything with inVision, so the screen was just a full screen JPG of the caller), but due to the tap feedback the user thought they were actually making a phone call.
In that moment the rest of the team on Skype would unmute their side and start with a simple "Hello", even though the sound was coming out of my notebook the user didn't actually realized it and started talking to my colleague as she was the sales rep the user intended to call in the first place.
Hope that clears that out!