Is saying whats up want to hang out to a girl creepy?

Hey guy's so here's a little background on my situation. So I just met a girl and want to get to know her better. Over the course of a few meetings at her workplace I told her she said seemed like a cool person and id like to get to know her outside of her place of work and she gave me her number. So I waited a day before calling her. When I called her today I made sure to call at a time after she got off of work so it wouldn't be weird. During the course of the conversation i made sure to tell her who was calling and to cut to the chase I asked in a casual manner if she wanted to hang out just friend wise. But before i could say anymore she said she was going to work.(mind you its 9:00pm at night and place of work closes at 8:00pm) But she did say to give her a call back at some point during the course of the conversation. After that I just said ok guess i caught you at a bad time. She said not to worry about it. We both said our goodbyes. So i guess what im wondering is did i do something wrong and what should I say to her next time when i call her back? Mind you i do eventually want to get to know this person better. Anyways all your answers to what I should do in this case would be great.