If you two have just met over the weekend and you would like to know each other better and you are in possession of his/her phone number. Then, call on Monday--at work if he/she gave you that number. The conversation should be as short as possible since Monday is a hectic day at work. This is a short call that declares your interest and ends with plans to call later in the week and make a definite date.

If you don't get him/her at work or the person is busy in a meeting etc then please make a point to call back and talk in person instead of leaving a message. However if you cant get him/her through out the day then don't go to bed Monday night without at least leaving a message that says you called and will call back later on in the week.

When you leave a message make the message short and sweet. If he/she doesn't have your number then leave it but if you are confident that he/she has your number then don't leave it. Ideally the message shouldn't even say 'Please call me back' or 'Give me a call when you're free' or 'If you don't call me back I'll sit her in a panic all night convinced that you hate me.' It should just say that you called.

Some people prefer to be called at work, some at home. Some people prefer to call and some prefer to be called. Some people like quick conversations, some people like to chat. Pay attention and figure out what type they are.

When your interest is to build a relationship then don't take it personally if he/she gets off the phone first. Don't go nuts counting out who's turn it is to call whom, but DO pay attention--if you're calling him all the time and he's never calling you, something is definitely wrong.