There isn't really a rule of thumb about calling girls but unless you can avoid it you should call a girl within the week that you acquire her phone number.
If it is longer than seven days then she might think that you are not interested in her. Being genuinely interested shows her that you are a discreet man who will not cause her any problems and does not mind if she has a boyfriend.
When to ask a girl for a second date?
Ideally when you get her number you should call her within 1 or 2 days. It all really depends on of how of an attraction there was between the both of you and how much that your really like her.
Ask yourself what transpired in your conversations with the girls that you have called previously. However if it was just a casual chat and you did not offer somewhere for you both to meet then it is not likely that she will call you back.