Call signs are an art and a science. They are a distilled essence of the character done at shorthand. More dead on or insiteful the better. Favorite ones in my groups would include:
Neo, short for Neopolitan, Face- former joygirl cybered to spy and for adaptability to please to order (she could be any flavor you wanted)
Sugar, Claire Sugarbaker, a conjuration specialist from the CAS swamps in LA. (Also went by black queen and Daylight-Runner)
Rascal, Erik Youngblood, elf latin hermetic combat mage, eco terrorist, playboy gambler.
Whyspre, Elf Japanese Smuggler, rigger, hacker, and all around tech guru.
Dark Secret, Rascals brothers hacker handle. (Group called him Klump due to his portly stature)
Smiley, phys ad Triad assassin who fought to get out of the life, faked his own death. Then formed this group to escape the mundane life of running a dance studio.
Jack, Dale 'Jackhammer' Mitchell, Smileys partner, a covert ops merc who is on the run from being framed by elements within the UCAS Army. (Based on Jack O'Neil on Stargate and some ATeam elements)
The group was one of my all time favorites and only stopped when one deployed and another moved to Iowa, they have winters ther according to him. Just isn't the same without them.