Call whenever you want, I just recommend you do it while your kind of busy so you cant spend much time on the phone, you don't want to seem like you have nothing else to do.
Use some callback humour (something that you teased her or talked about that you can say) e.g. "Hey barbie" if she likes to dress up. anything really to just make it more comfortable.
http://www.pualingo.com/pua-definitions/callback-humor/
Quote:
We talked about where we are from and since I am not native, she told me she loves my country and wants to travel there and we talked about traveling etc... for a while.
something from this maybe.
When she answers don't think of it to serious, just be in the mindset of your testing her to see if she is fun to talk to, then invite her out if so.
If she doesn't answer its up to you, forget it or leave a voicemail.
If you decide to leave a voicemail make it something open ended for example:
"Hey HB, There was this this really important thing I wanted to ask you but.." and hang up.
Or something fun. here's a good example
view-next-topic-vt12946.html?view=next
You don't have to follow what I said I just put it in case you needed some ideas
Let us know how it goes.