When you are talking to others in the same riding group you are generally talking about the shared experience so you are concentrating on what is around you.
When you are talking to someone elsewhere your mind starts to picture them whereever they are and you also start picturing what you are discussing. This distracts you from the highly IMPORTANT job of riding/driving.
Some drivers/riders can keep their concentration in the here and now but most can't.
If they are having an argument on the phone, STAY WELL CLEAR as they are apt to take their frustration out on the next thing they see.
Cellphone use in a car (even handsfree) is banned here in the UK, but rarely enforced.
I used to pass 20-30 people every day who were yakking on the phone.
Jeez, if you can't possibly live without taking/making the call then pull over and do it.
Phone/text drivers are as bad as drink/drug drivers and should be penalised accordingly.
IMHO as always.