The first thing here is more of what you 'shouldn't' do! You shouldn't keep calling her. By constantly calling her when she's not returning your calls, you are appearing to be desperate and needy, neither of these is particularly attractive attributes to women. If she's not returning your calls you really need to consider why she's not interested in talking to you.

Is this a girl that you've been dating? If so, think about the last time you were together. What was said? Did you do anything that would have her mad at you? How mad was she? It could be that she needs a little bit of space and time to calm down before she speaks with you again!

Did you break up and you're calling her to try and patch things up? If this is the case, why did you break up? Does she want to date someone else? If she does, then you need to let go now. It's possible that she may come back to you if things don't work out with this other guy and if you still want to date her at that time, then perhaps things will go well. It's also possible that she won't come back to you at all, that she's move on emotionally as well as physically and so the best thing you can do is to let her go and find someone else who will appreciate you more.

If this is a girl that you haven't dated but would like to, then you should stop ringing her immediately! Women like their men to be a little less insistent. Having a guy who wants you is one thing, having one that won't leave you alone is another -- so constantly ringing her is going to begin to make her feel as if you're harassing her. Don't do it! Stop ringing her and see what happens. It could be that you won't hear from her, in which case you have to accept that this relationship wasn't meant to be, and then find a girl who will return your calls. On the other hand, perhaps once you stop dogging her, you'll find that she does ring you -- if only to find out why you stopped calling!

Accepting that a girl you want to connect with isn't interested enough in you to return your calls isn't easy. In an ideal world any girl you're attracted to will be interested in you and will immediately return your calls. Unfortunately that's not how it works and so if you call a girl a few times, ask her to ring you back and she doesn't, you have to accept that this isn't going to happen. Think of it as being her loss and another girl's gain as you start looking around for another girl to date.