He was born off screen and wasn't seen on air until a few months after his birth. He was assumed to be born around December 5th or so based on Pam's online blog.
Phillip is named after Pam's grandfather. He is the younger brother of Cece Halpert who often calls him Pull Up instead of Phillip.
The character of Philip was brought on the show because actress Jenna Fischer got pregnant.
Phillip is also the name of Angela's baby that she had on the episode Jury Duty. Phillip was Jim and Pam's second baby. Pam went on maternity leave in Gettysburg and came back in Secretary's Day.