Why did the cops ask me more questions?

The police came to my door to ask if I recognized a phone number. I didn't recognize it and I told them, but after that they began asking me for my personal details such as phone number, name, date of birth, and even where my family was at the time.

Why did they do this if I already told them I knew nothing? I feel like it's possible I looked suspicious, but still it's weird that they asked me. I mean is this standard protocol?