From the DfT website advice, hands free (including headsets) are fine provided the phone itself is in a fixed cradle in the car/van etc. (not in pocket or just lying around) Further, you have to be able to answer the phone by either auto answer, voice activation, or a single touch to a button. The new regulations specifically say that drivers should not hold the phone in their hands to answer a call.
The advice on starting a call is a bit wooly, but boils down to an advisory not to do so, until the vehicle is at rest with the engine off.
The Royal Society for the Prevention of Accidents have researched this general area and state that a driver chatting on the 'phone of any sort is 4 times more likely to have an accident. We have all travelled behind someone chatting, and noticed them get slower and less attentive, and I am sure that this is true, although subjectively the person on the phone will think that therer is nothing wrong.
I think I go along wit the general DfT advice to divert to voicemail during driving, or have a passenger use the phone and take a message. I think this option is best for the safety of self and other road users.