I hum or sing showtunes, sing songs, immitate characters, or sing catch-phrases from commercials. Then, at the main point I subsitute whatever word in the lyrics with "Hello" sung.
Usually, I have a second to customize it to whoever's calling and if I want to talk to them. Example: a slvt is calling...
"From the Valley of the Jolly - Ho, Ho, Ho... He-llo-o."
or,
"There's some HELLO'S in this house, there's some HELLO's in this house."
A hottie:
"Oooh, Aaah, OH! Poppin'-fresh Hellooo"
Random:
"Because, because, because, becaaaaaauuuse.... because of the wonderful things Heh-lo!"
Random:
(Lyrch from Addam's Family voice) "Youu Raang."
Random:
*in Johnny Cash karaoke mode* "I slipped and fell into the wet concrete HELLO."
Also, I have a simple VM message that I must've said 500 times before I recorded it, so I can answer the phone and mimmick it:
"Heeyy, it's Vulpine. Leave a message." *hang up, wait for call back, if none: call back and ask them why they called and didn't leave a message/what they wanted* The alternative is to see if they go for the fake "BEEEEP" and start leaving a message.
People get confused when you answer: "Diga!" or "Moshi, moshi."