Well, I'm a definite minority here, but I do answer my landline, unless I'm in the middle of something important like dinner or bathing a child. What I don't answer is my cell, unless I'm away from the house AND in a position to talk (unless it's DH, because he won't call me when I'm out unless it's something important that won't wait.) Why, you ask? Money. I'm extremely frugal with my fixed expenses, and phones are a big one. We also don't have caller ID or call waiting.
We don't have outgoing long-distance service on our landline, except for being able to dial 800 numbers. We both use pay-as-you-go cell phones, so long-distance calls on those cost us $.25/minute, just like any other call. If we want to make long-distance calls at any time other than an emergency, we use our landline with calling cards from Sam's Club that cost well under $.01/minute.
Now here's the annoying thing. I'm phobic about making phone calls; it takes every bit of willpower I have to dial the phone and make a call. If I call you using my calling card the number that will show on your caller ID is one that comes from the service's router -- with the one I have at the moment it usually shows a number in Denver, while I live in Missouri. If you have a voicemail message service I'll use it, but the number I'll leave is not the number your machine will automatically record; it will be my landline, because I don't pay for incoming calls on it. Therefore it is a real problem for me when people use ONLY callerID to screen calls. They wait for me to call them back, but I won't, because it's so hard for me to get up the determination to do it. If I've left you a message, I'll wait for you to call me back, even if it takes years. (BTW, my phobia isn't all that unusual; there are quite a few DISers who have it, too. There was a poll thread a while back.)
To compound the problem, members of my extended family tend to input only my cell number into their cell phone memory, in spite of the fact that I've asked them not to use that number except in emergencies. So they forget where they wrote down my home phone and call the cell instead, which I normally won't answer because I don't want to pay $10 for the call. It's a vicious cycle.
Just so you know, there are a lot of older folks on fixed incomes who use calling cards and prepaid cell phones, too. (I'm not older, just frugal. I'm 46.) Screening by phone number costs them money, often money that they can't afford to lose.
Oh, one other thing. I don't call cell phone numbers unless I am SPECIFICALLY told to prefer that number. I think it is rude to possibly force someone to pay for an incoming call if they did not specifically offer to do so. Since I'm generally not privy to the details of others' phone plans, I try to be polite and avoid having them pay for the privilege of speaking to me.