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?

Could be, plawrence. Another possible explanation: The reason Phillips was of use to the Corleones was that he was on the NYPD's "Mob Squad," or its equivalent in that era. Since his nominal job was to help the cops (not the Corleones), and since he supposedly knew more about the Corleones than other detectives, he was detailed to go to New Hampshire to lean on Kay and her family. He really had no choice: he couldn't very well refuse without raising questions about his loyalties. In short: he had to do it, but I don't think that it meant he'd turned on the Corleones.

I just reread the passage. I think Det. Phillips plays the good cop, to Det. Siriani's bad cop.

Phillips politely asks Kay some leading questions. Siren asks the tough questions, and tries to intimidate Kay with details of her sexual relationship with Mike.