There is no answer that covers all circumstances. LACs, for example, tend to care more than larger U’s about finding students that are a fit for them. Talking with a student on the phone can lead to some positive feedback to admissions. Its not they’d be offended if they don’t talk to you, but its a potentially missed chance to boost your app.
But by and large these are recruiting calls. People hear the numbers from colleges like Berkeley or Harvard and worry they won’t get in anywhere. Actually most colleges admit most of their applicants; St. Mary’s for example admits about 70%. They know that if you are a student they admit, you are going to have lots of similar choices. An outreach program of having students contact their applicants, they figure, will raise the odds you pick them.
So who are they recruiting? Only the colleges know. Some may call just about everyone who applied, figuring it helps raise their visibility. Some may call stronger applicants only. Some may call based on zipcode, trying to get more students they figure will be full-pay to accept.
But these calls can be helpful to you in understanding the school. Colleges differ in more ways than many HS students imagine; class size, housing, career center, advising, etc. With a current student on the line the call is your chance to get a better idea of what it would be like to attend. I wrote a post a few years back with some questions I found online; it may give you some ideas of things you’d want to ask. That post is at http://talk.collegeconfidential.com/college-admissions/431878-phone-calls-from-students.html