Difference in behavior. On phone & in person?

I am a 19 years old girl, considered extremely beautiful. I have this administrator in my college who is around 45 years old. When in the office or professional enviornment, he is alright with me and even usually IGNORES me when someone(other admin or manager etc) is around. Though, in college, when we pass in the hallway, we surely look at each other and would continue staring for 3-4 seconds. Other than this, once he said something very flirty to me when I was near him but otherwise he would ALWAYS ignore me or just look at me & not talk to me and if talks, its ALL professional. But
But, when I talked to him on phone from my home, I had to talk to him to discuss my paper issues. On the fone, his voice is SOOOOOOOOOOOOO romantic & like when I will tell him that I am speaking wit my name, he will YESSS ROSSSEEEE... & things like this. Once on the phone, I told him that how I couldn't come to the university so he asked me why & I told him & he started talking in detail and making jokes and making me laugh on phone rather than solving my problem. Once, I called at the office & couldn't recognise his voice so I asked who is this speaking and he said that, "Ohh, you are not recognising me, You should recognise me." I ignored his sentence and discussed my professional problem and asked him that "Is it possible to solve it?" and he replied, "I can solve everything FOR YOU, but you should atleast recognise me."

Why does he behave in this way on the phone & different in person?

Other than telling everyone that you consider yourself extremely attractive, you are asking why he acts this way? How about because he has to be "professional" in a work environment but feels talking on the phone is less of a professional situation? Could be attracted to you but feels he shouldn't show it?

Are you interested in him?

I don't think she would have opened this thread if she weren't interested in him... at the very least, she is interested in the attention he gives her. Yes, OP, he is attracted to you and the reason he acts this way is what Sixpacj said, "he has to be "professional" in a work environment but feels talking on the phone is less of a professional situation". I personally wouldn't trust him, FWIW. But you're probably excited at the idea of a "forbidden" passion or something, so whatever.