Because as with most conversation it depends who you're talking to, I've separated it by who the Chatty McChatterson is. Also, for the purposes of the first three questions, let's assume you're not necessarily in the middle of having open heart surgery, but are still busy enough - or about to be busy enough - that you really shouldn't be, or don't want to be, on the phone.
If it's a friend or family member?
If it's work related? (Client, coworker, business associate, etc.)
If it's someone you don't know well/aren't close with?
Let's suppose you're at an important family or friend gathering, and have a work call you technically have to (or at least really should) answer. What do you do?
In general, do you take calls while you're otherwise occupied?