I take a long time to call back (much to my mother’s dismay) because I just don’t realize I’ve missed a call.
a) Almost all of my chatting is on the computer, whether it’s through emails, website, IM programs or games. That’s a massive chunk of what I do, my phone is used more as a clock/calculator/alarm than it is as a phone.
b) I don’t get any significant notice that I have missed a call. If I miss a text, and I check my phone later (to check the time, of course), my front LCD screen is completely lit up saying “1 missed message”, and when I open up my phone it opens right up to the text. If I miss a call, what do I get? A tiny little icon that I typically mistake for the icon that indicates my phone is on vibrate mode. That’s all. So it may be days until I actually look at the icons and realize I’ve missed a call.