Let me get this straight. This kind of stuff happens ALL THE TIME. Girl calls, guy calls back, chats a bit, tells her to call back somewhere in the week, girl calls back a few days later, guy doesn't answer, doesn't call back. Girl is confused.
Before you get the girl condamned because "it's so obvious she's in love", WAIT. The story doesn't end here.
Sometimes it happens that a girl wants some sex. Awful but true. What's better than a booty call when the uterus quivers? Girl grabs the phone, dials a number and waits for a satisfying session of greco-roman fight in the bed. No calling-back. Frustrated uterus.
Let me explain you something that you guys don't seem to understand well. If the deal you have with the girl is that you are just sex buddies, please understand that when we call you, it's because we need to fuck really bad. We're really manly about that. Sure we are nice to you and we look at you with kindness because we like to spend time with you. And let's admit it, we like the way you make love to us. But it doesn't mean we're going to fall in love with you if you answer that damn phone.
And if you don't answer, you keep the girl wanting more. Awful but true. You're keeping her calling more and more and more. We all go:"Oh please. Not the "Ignoring trick" AGAIN." And that's bad because we just hate you for that moment. But if you call back and just say:"I'm so sorry, I'm busy right now, I have something planned, but I'm gonna call you back on (insert day here).", I swear to you she's gonna be quiet for a week or at least for a few days. Touchdown. But be sure you call the day you said you were going to call. If not, she's going to be upset AND she's gonna call you to make sure you know it.
So overall, that's it. Girls aren't that complicated. You guys, are.