A graduate of Washington DC's American University, the energetic native New Yorker did well on the stand-up comedy circuit, even performing his act on "The Late Show with David Letterman" (CBS). Though comedy was his primary genre, Callen held his own with a 1998 recurring role on the second season of the far-from-funny HBO prison series "Oz", playing a prisoner who tutors his fellow inmates and inevitably comes across some overly tough students. That same year he was featured in an episode of the NBC sitcom "NewsRadio" and in 1999 returned to the network as a radio host, this time playing an obnoxious morning show shock jock tormenting radio psychologist "Frasier". In 2001 he joined the network as a series regular, playing the straight-shooting best friend of Adam Schwartz (Breckin Meyer) whose active internal life provides comedic fodder on "Inside Schwartz".
Callen's big screen pursuits include a role in the disappointing 1999 comedy "Wirey Spindell", written and directed by as well as starring Eric Schaefer. He was also featured as the reprehensible Brian in the independent "Kiss and Tell" that same year. In 2000, the short "Jack and Jill" (written by and starring Callen) won the Audience Award at the Hamptons International Film Festival.