cell phone.
"'Sup kids? Gumi here. I'm probably off helping Onii-sama with his tech fail, working, harassing either Reki, Luka, or Lills, or just doing something stupid around the Institute. If you need to get a hold of me, just leave me a message with your name and number. Oh, and, try to make it funny. I like that."
[BEEEEEEEEP.]
Supports Voice, Text, Internet.