Dear John1988,150 years ago
Well, surely she said more than just that? Maybe something came up unexpectedly in which case you could say, "That's OK, I'll check in with you another day." The next time if she gives another excuse then she might not be interested for one reason or another, but if you're still interested, leave it up to her to contact you by saying, "I understand, if you ever want to get together give me a call." I wouldn't contact her again.
Dear John1988,149 years ago
I just don' get the games people play. If she didn't want to go out with you again, why ask you to call? She didn't have to say anything about the date or give you her number if she already knew she would be busy the next day, or wasn't interested. What happened to good old honesty?
I would say the ball is in her court now, your number is on her caller ID, and if she was sincere about wanting to see you, she should be calling you when she isn't too busy.