If you were expecting me to phone you back and haven't received a call, you might want to consider the following:
I frequently find people leave messages on my answering machine asking me to call them back, but they don't leave their phone number, they say it in an unintelligible manner, or they say it wrong. I can't return calls if I can't work out what number to ring.
I often get people asking me to return a call to them at their work, sometimes without even mentioning that it's a work number, but they don't leave their name, making it impossible to find the right person to speak to.
If you've asked me to call you on a mobile phone, it's not much good if you keep it switched off, spend most of your time in areas without reception, don't have it with you, or have batteries that only last for half a minute. I keep mine with me at all times that I'm able to answer calls on it, turned on, with a properly charged and working battery.
I'm not keen on making expensive calls to mobile phones, or long distance, for things with little profit in it. e.g. My website lists prices, yet someone will want me to spend $4 calling them about a quote for a job worth only $10, that they might ask me to do.
If you're only available at certain times, please say so. I can call back during business hours or after them.
If you haven't received a call back by the next day you should probably consider that I've been unable to reach you for some reason, call me again.
If you want a return call, make it easy to do so. I don't put a great deal of effort into returning phone calls to people who are difficult to reach. They'll have to call me back.