You don't have to call them, but by sending you their phone number it gives you the chance to make contact beyond e-mail if you are comfortable with it. I generally send a couple of e-mails to get a feel for the person before I make the first call.

If someone is worried about that, call from a pay phone or office phone, or make your phone number blocked with your cell phone.

I have a buddy who went to sell some motorcycles to a guy and they got to talking and now the buyer calls this guy all the time to invite him to dinner, and over for drinks etc. My bud tells me that this guy was a racist, and just flat obnoxious. He has tried nicely to tell him that they are not friends but next time he is going to tell him off permanently.