This is DEFINITELY a sign of low interest level. (Especially if it came while you were trying to ask her to hang out). If this is so:
Her : LET ME CALL YOU BACK
95% of the time means: I DON'T KNOW WHAT ELSE TO SAY, BUT THERE'S NO WAY IN HELL I WANNA SEE YOU, SO I'LL JUST TELL YOU I'LL "CALL YOU BACK", AND HOPE THAT YOU GET THE POINT ONCE I NEVER CALL YOU.
**** that Persuing bul****. There's a limit to everything, or you may be looking at bigger problems down the line if you really like this girl so much. Her not being interested and trying to AVOID you by telling you she'll "call you back" many times over and over and you KEEP calling her = NO CHALLENGE. The poor girl may finally give in, and become your girlfriend or something for whatever reason. We all know that girls can go out with guys that they're really not all that interested in. If you're just trying to have FUN with her and beat it a couple of times, then whatever, you can try a few times I guess, I wouldn't personally but you can give it a shot. I'd call maybe 1 more time total, if the same thing happens, move on.
If she finally gives in after you keep on calling her, and you're like obsessed with this chick, trust me, they'll be problems down the line. You'll notice that you're the one who cares about the relationship the most and she always looks like she could give a fvck less. This is because SHE NEVER LIKED YOU IN THE FIRST PLACE! It's all up to you man, but be very careful, since you sound like you're puttin a bit too much importance on this one chick who's so flaky.
Now, like anything else in life, there may be exeptions to this, but you're the one who knows for sure. My honest guess is, 95+% of the time, this is true, she has LOW interest level (or none). Watch her tone, would a girl who's really interested in you tell you "shell call you back" after she gets her CHANCE! to hang out with you?? Think about it. Would you tell some chick you would LOVE to see that you'll call her back, and then keep doing it an never call her?