When a friend was complaining one night about never getting any messages on his answering machine, we decided to make his wish come true…with a twist. We left the following messages one after another:
He went nuts trying to figure out what happened to “Message 4”. 
As for sending a text message (to another phone/beeper/etc.), how about, “General Protection Fault”.
FNORD
Error: User is a c*nt
or
Calling…
911
With that second one, it’s fairly obvious it’s a prank quite quickly but the few moments of blind terror are priceless.
I’m usually quit good at this.
“Don’t turn around.”
Lots of personal info displayed in an “I-know who-you-are” manner. Particularly good good in the event that any of your closer freinds might have entrusted you with their SS or PIN numbers for whatever reason. Heh heh.
eh… that’s all i got for now. I’m tired.
flupppppppppppppppppppppppppppppppppppppppppppppppppppppppp
“Feed me.”
“This phone is set to self-destruct in–[connection lost]”
“Bingeley bingely beep!” (obscure Terry Pratchett reference)
SURF’S UP. Unless, of course, you are both surfers, then it would make entirely too much sense.