I have a habit - that of ignoring the phone calls from the numbers I do not recognize.
Usually what happens is, when I am in good mood, I tend to pick up calls from any number, and engage in a warm chat.
On the contrary, when I am not in the best of my spirits, I most definitely decline calls from numbers I do not recognize (read, numbers not saved in my phone address book). And in many cases, I disconnect the calls from my acquaintances too, only to return call at a later stage.
Not to mention, I disconnect calls when I am busy as well, again to call back at a later stage.
And when someone lets the phone ringing, without picking it up, that's a heart breaker!