The following are characters who have been voiced (usually via a phone call), but never physically seen.
In the episode Halloween, Michael speaks with Jan's secretary Sherry over the phone regarding who Michael planned on firing that day. At that point, however, he had not decided and tells Sherry this. She reiterates to him that he must pick someone by the end of the day. After assuming she hung up, Michael says he wishes he could fire Sherry, however she is still on the line.
The voice actor is uncredited.
In the episode Launch Party, Ryan opts to have one of his talking head segments in Thomas Dean's office because it is larger. From off-screen, Dean yells at Ryan to move it to either the hallway or his own office. The size of the office, and Ryan's visible fear indicates that Dean is in a higher position in the company than Ryan. He was voiced by Scott Damian.
In the episode Branch Wars, after discovering that Stanley is planning to leave Scranton in favor of the Utica branch (where Karen is Regional Manager), Michael attempts to steal away Karen's top salesman, Ben Nugent. The phone call does not go well however, as Ben recalls Michael having run over an employee with his car. Additionally, Ben says he has heard that the Scranton branch is even worse than the Camden branch. He is named after Mindy Kaling's real life boyfriend and was voiced by show writer Lester Lewis.
In the Producer's Cut of the episode Branch Closing, Meredith spends much of her day trying to remember which co-worker she made a pact to have sex with upon the closure of the branch. Meredith is later shown on the phone talking to a Gary Trundle, a former warehouse employee who she had made the pact with.