Hello! When you say call, do you mean FaceTime? You can only facetime another apple computer that has facetime installed on it. It has to be an apple product to work.
To call a computer, you 'phone' the persons email address rather than their phone number and if they have the facetime app on the computer, then it should come through on that as well as on their phone etc.
Just make sure your friend has bought the app, I think its about 69p to buy it on a mac (which is annoying as its free on an iphone/ipad!)