I will approach even of they are on the phone.
As skjoldr said, sometimes they arent going to pay attention to you and will carry on.
But, the more calibrated and interesting your approach, the better your fundamentals and looks, and (if possible), waiting until the time is “right” and you will have much better odds of her putting the phone down.
Have you ever heard of the OODA Loop?
Observe
Orient
Decide
Act
This is a process we go through thousands of times a day, often in milliseconds, that allows us to make decisions. I personally prefer my own version: REDI.
Read
Evaluate
Determine
implement
When you approach her brain is going to automatically “observe and read” the situation.
Shes on the phone and a homeless looking dude comes up out of no where and says something to her… what do you think her brain is going to observe/read about this?
“Pest! get away fast!”
Shes not putting the phone down.
Now she sees a dude dressed nice, well groomed, he catches he eye first and smiles, then he approaches. No go through what you believe her thought process in terms of Observe/reading is? Is she more likely to give him a second? Of course.
You approach, she tells her friend on the phone “hold on a second.” Now shes going to automatically orient/evaluate HOW you approached and WHAT you said.
Picture yourself sing for directions. “hey do you know where PLACE is?” She has evaluated that you want directions and she can be socially courteous and quickly give you directions and get back to her phone call.
Now instead picture that you asked her something that makes her intrigued or curious. “This is random but has anyone ever told you that you have interesting energy?”
She now gets to decide/determine her options. Because you intrigued her, shes often going to choose to follow up on that as opposed to getting back to her phone call if you simply asked for directions.
Then she acts/implements her decision by either going back to her phone or responding to you.
Do you think her mind is now on the phone call or did you lead her thoughts into an entirely new place?