He said in the interview that anytime he drives in his car, he would not talk on his phone or listen to the radio. It was complete silence for the duration of the drive. And most of his best work and ideas came from these "silent drives" because he was forced to be creative and use his imagination to keep himself entertained.
Driving is the classic 'zone' activity. As such I question if it's even a good idea to attempt to think and be creative when driving or if that's (if what Spielberg is saying is true even 'best worked because forced to'). Not that having a phone call isn't distracting from driving either. It is. But when you drive I just am not sure it's when you want to go back and forth in your head with some heavy thoughts which might make you less alert (and by less alert I mean even less than with a phone call).
The reason for taking phone calls in the car is because it's typically very time effective and a great (and effortless) way to pass the time.
There could also be other reasons why he didn't take phone calls. For example I don't like to take business calls because I can't take notes or note things I need to do. Talking to a relative that's another story. In that case it's great a way to pass the time and let's say fulfill an obligation. Or he might have almost gotten into an accident once and swore off any future calls. Or the main reason is 'I want down time but I need a way to explain to others why I don't talk in the car and this sounds better'. After all Spielberg is a creative person so why not a creative spin to something he does.
The story and the reason people say what they do often differs from the real reason (which would sound less impressive). Possible this is just a spin he put on it.
Really quiet or electronic music can work too though. It's people talking or singing lyrics that makes thinking difficult for me.
Pretty similar to coding, actually, except you're mostly stuck in front of a computer for that.