I've always had a problem with this. Girl gives phone number, I call and leave a brief voicemail, she doesn't call back. I let 2-4 days go by and want to contact her again. If I follow up, I usually do this back on the dating site since the phone didn't work. So my follow up message will go something like:
"Hey, I left you a voicemail, but I didn't hear back. I'd still like to get together." <-- wreaks of neediness, I know...
...so I try some variation:
"Hey, I left you a voicemail on Thursday, but I'm sure your job keeps you busy. I'd still like to get together sometime." <-- still wreaks of neediness.
I can't figure out how to send a follow-up without it sounding needy. Most of the time I simply don't follow up because I can't find a good way to do it. One phone call, no call-back, I just drop it right there and move on. But I think it would be nice if I could find a fun non-needy way to follow-up, in case something legitimately happened and she couldn't call back.