One funny phrase to use when answering the phone is, "Roadkill Cafe, you kill it, we grill it, how may I help you?" This phrase uses word play and rhyming while incorporating a bit of classic shock value and nonsense. Another comical phrase might be, "Hello, caller, you're on the air!" This greeting makes the caller think that she may have accidentally called a live radio show.
Another witty way to answer the phone is to mimic a disconnected signal by saying, "beep, beep, beep." Or, try answering the caller like an answering machine has caught the call. For example, pause for a moment after answering, and then say something such as, "Hello, you've reached [name]. I can't come to the phone right now, but if you leave a message, I'll get back to you," followed by a "beep" sound. Another option is the popular, "Hello, Grand Central Station!" This play on words uses a familiar and recognizable phrase to confer the present atmosphere -- in this case, busy.
Perhaps the best way to achieve a humorous telephone greeting is to consider the audience. If the person on the other end of the phone is a friend or family member, play off of familiar knowledge to construct a phrase to amuse that specific listener.