Shoob, it's all about the girl's security. Some will also refuse calls from payphone and some will not even answer blocked calls so, you were in luck she picked up the phone.
If she fall on a psycho who cause her trouble, she need to have a way to point whoever in the direction of the problem.
Also, lots of idiots simply book fake appointments, wasting the girl's time and sometime money to go to meetings that the guy never planned to attend to. A phone number cuts down on this since not many jokers will buy a prepaid phone simply to do that.
It's sad for us the girls have to proceed that way but, they are responsible for their own safety. All we can do is either accept that policy or not. Mentioning this is the girl's policy is ok but, complaining about it isn't doing any good: she set the rules, we accept or not, end of story.