Some of the researchers created some or all of the scenarios you suggested and they did show that in some, conversations with passengers and listening to the radio could be as distracting as using a mobile phone.

The same research also demonstrated that distraction levels were almost identical when using a hand held or hands free mobile.

What appears to be beyond dispute is that a driver is significantly less likely to have an accident and to drive more safely when not using a mobile.

There is very little that can be done to stop the distraction caused by passengers but that doesn't mean that we shouldn't take measures to reduce the use of mobiles that can be enforced as well as detecting their use should an accident occur or another driving offence be committed.

The assertion in an earlier post, "If it's the conversation that distracts then you'd have to ban talking to passengers as well?" is spurious.