Rotorspeed
The fact that a mobile phone was being used to exchange text messages while flying has to be a contributory factor in the accident. It is an indication of someone who is prepared to bend the rules, push the limits a bit, and to a certain extent compromise safety.
Since I hold operating licences for operating two way radio equipment in any vehicle (including while driving), ship or aircraft, I feel confident in stating that using a mobile phone to text or speak while flying will lead to a degradation in performance.
The phone may or may not have been in use at the actual moment of impact, but it was a significant factor in this accident in my personal opinion. In the case of my phone, messages repeat every couple of minutes and flash on the screen until positive action is taken to acknowledge them. That could be sufficient distraction to divert a pilot's attention from the view outside for those vital couple of seconds...
It is also significant that many car accidents attributed to mobile phone use also involve the exchange of text messages, rather than people just talking on the phone.