Listening to music is passive (yes, even if you happen to be singing along, as you're just parroting memorized lyrics), whereas during a phone call, you have to

Listening to music is passive (yes, even if you happen to be singing along, as you're just parroting memorized lyrics), whereas during a phone call, you have to focus on what the other party is saying, and formulate sentences to answer them. That affects your attention on driving.