Here are some responses I used…
When they ask for my mom I say “Which one?”
When they asked for my dad in the middle of January w/tons of snow on the ground, my brother absentmindedly said “He’s mowing the lawn.”
“No habla english SORRY.”
Of course NOW I feel bad doing this because for 2 hours I was a telemarketer and after the 2 hours of work I ran out screaming…that was the WORST job I’ve ever had!!! 
::answering phone::
“You called back! I knew it, Jessica, you really do love me!”
“Um…hello? This isn’t Jessica.”
“You bastard! How can you sit there and play with people’s emotions like that?”
::hangs up::
tisiphone wrote:
Well, the OP did ask how you answer the phone when you want to screw with people…
“Thank you for calling Rock-n-Rolga’s twenty-four hour billiard hall, tattoo parlor and Marijuana emporium!”
No one’s been offended by it. Even my own mom (who knows I’m uninked and don’t touch pot) just said, “So that’s how you’re paying for the spring semster!”
“City morgue, which stiff would you like to speak to?”
“Listen,I’ve told you once, I’ve told you a thousand times, don’t call me at work” then hang-up.
“The phone service you have just dialed is unavailable. Please replace the receiver and hit it with a hammer”
Good morning. How would you like your debauchery today?
My favorite was Steve Martin’s in L.A. Story:
Steve: “Hi. You’ve reached [name]. I’m here right now, so start talking after the sound of the beep.”
[holds beeper up to the receiver] beeeeeeep
caller: [long pause] “uhh… hello?”
Steve: “Hi.”
…
–sublight.
I sometimes answer the phone with, “Mishell’s House of Pain, how may I spank you?” Or “Pepe’s Casa de Queso, que pasa?” I’ve never had a mishap with those.
However, once when I was in high school, I answered the phone with “Heaven, God speaking!” and it turned out to be a woman from our church. A very, very devout woman from our church. She had called to tell my mom she was on sandwich duty for a funeral. She didn’t say anything, but I could hear how offended she was from the tone of her voice…hehe.
I’m not sure if this falls under the OP, but what the heck. One of my fav. ones is from Calvin and Hobbes:
(Ring) Calvin: Hello?
Person: Hi. Is your father home?
Calvin: No.
Person: Could you take a message please?
Calvin: Sure. Hang on while I grab some paper and a pencil.
(Blows up balloon, then pops it next to receiver)
Calvin: (POW!) AUGH!! I’ve been shot!
(Next frame)
Calvin: I hate taking messages.
I’ve never tried this, cause I’d like to avoid getting cops and an ambulance sent to my house… =)
Calvin: Hello?
Caller: May I speak with your father, please?
Calvin: Heck, you don’t need my permission! Be my guest!
Hangs up phone and rolls eyes: What a weirdo.
One time when I dialed a wrong number the phone was answered by an obviously irritated guy yelling
YES GRANDMOTHER?!?!?!
First what a few friends say:
Conjunction junction, whats your function?
24 Church of Elvis, have you accepted the king into your heart?
Now, what I say.
[Hitchcock Voice] Gooood Eeeeeeeeevening[/Hitchcock voice]
When I was in high school, I would answer the phone “Ching Wong Cleaners.”
If I knew it was my sister’s boyfriend I would say “I hate you Frank. Quit calling!” and then I’d hang up the phone.
Not to long ago I answered the phone with a simple “Bueno?” Unfortunately it was someone from Straight Dope and she said “Why the fuck did you answer the phone like that?”
This is best when said in either a deep construction worker voice or a breathy phone sex operator voice. I usually opt for the latter. Bob’s sperm bank, you squeeze it we freeze it.
I did the “Heaven, God speaking” thing once, and it was my friendly neighborhood Mormon missionary sister on the other end.
I used to have an OGM on my answering machine that said
“You have reached the Society for the Molecularly Disadvantaged. Our office is closed right now. If you have experienced loss of covalent integrity, please call our emergency hotline. Otherwise, leave a message and we’ll get back with you as soon as we can.”
I had to erase it and use a “normal” message when I was job hunting, and now I share an apartment, and a phone with my mother, so I can’t use it anymore, but it was fun while it lasted.
Sometimes just to mess with people my answering machine answers as follows:
Hello <pause> Who? <pause> Speak up I can barely hear you! <pause> What? <pause> You’ll need to speak up much louder, I’m having problems hearing you! <pause> No, this isn’t Pizza Hut. I think you have the wrong number! However, if you wish to leave a message, do so after the beep… <beep>
People have told me how they were screaming trying to be heard. Try it, it works great!
My folks kept getting calls in the wee small hours for a woman named “Charlotte” No matter how they tried to tell the callers that they had a wrong number they kept getting the late calls. So one late night, when my dad ansered the phone yet again he said, “Oh, she went out with Bob” Suprisingly, they never got another wrong number for “Charlotte” again.
Answering in Spanish used to work pretty well and answering in an East Indian accent usually gets people apologizing and hanging up.
I once put a message on my answering machine that went like this:
"Hello and thank you for calling the 24 hour trivia line. You too can win fantastic prizes by simply answering today’s selected trivia question.
Today’s question: How long is a cubit?
After answering the question please leave your name and number so that we may contact you and let you know what you’ve won."
I answered the phone one day and it was my mom. She said she was happy to have finally gotten a hold of me… seems that she kept getting this “trivia line” for the past couple of weeks…
People at work threatened to hurt me if I didn’t change my messages like…
"Hello? (Loud Music playing in the background)
Hello?
I can’t hear you… just a minute… Pete! Turn that crap down! (music goes down)
There… that’s better… now that you can hear I can tell you that we’re not home. You know the rest.
BEEEEEEEEEEEEEEEP!
24 hour abortion clinic. You rape them, we scrape them.
Of course, that was a lot funnier 20 years ago.
CALLED - Can I ask you a question?
CALLER - Sure
CALLED - Whats approx. 8 inches long, hard to the touch, and goes dead in your hand?
When we bought our house we started receiving TM calls every day. One time a female was selling alarm systems and I tried the heavy breathing and asking what she had on. She went right along with it, I asked what she had on underneath, she told me. I could not carry it to far with my little girl in hearing distance so I just hung up. I’m sure the TM got a laugh out of it.