Where is the line between creeper and persistence.?

I met a girl a weekend back that we seemed to hit it off. I facebook said hi to her but didn't want be lame and try and start a thing with her on the internet. so I got her number from her and called to ask her out over the weekend. She answered and said hey I am in the middle of something call me back later. So i waited until that night and called her back. she didn't answer. I figured she wouldn't give me her number if she didn't want me to call and she seemed happy about it. Obviously I am not going to keep calling but I might try again in a few days. Is this a situation where, given time and space, persistence could pay off or am I just being a creeper by calling again?