He's Sonny's "inside man" on the police force, and shows up three times in the novel.

The first time, it is he who calls Sonny on the phone to tell him his father's been shot. It seems he was in the "lead car" of detectives who responded to the shooting.
Then, at the hospital, it is Detective Phillips who tries to protect Michael from McCluskey ("The kid is clean, Captain...He's a war hero...)
But after Michael kills Sollozzo and McCluskey, it is the ubiquitous Phillips who (along with his partner, Det. Siriani) shows up at Kay's house in New Hampshire and really leans on here for information about Michael's whereabouts.
Seems to me that Phillips, who really gets around, had a change of heart somewhere about his allegiance to the Corleone family. Or was it just more careless plotting?