Just in my opinion, this guy sounds creepy. Some people seem to have a weird instinct for when people feel vulnerable or lonely, he sounds like one of them. Run for the hills. I also would be making it clear to my sister NOT to give my phone number out to anyone, unless she has asked me first. Mind you, she perhaps thought it would be ok since you had known him before. People change over the years and I am not one for revisiting old acquaintances/friends. I don't see the point,surely if you are good friends,you stay in touch over the years. I blame Facebook for alot of this,it doesn't always end well.