[F.A.S.T]

Is there any way I could find out all of someones Contacts by their phone number?

[CyberStrike]

With just a phone number and nothing else? Wow umm I would venture to say no.

[Bouzazi]

Requires access to the phone to retrieve the list.
You can send him a malicious MMS if he's vulnerable to Stagefright exploit -- Just an idea...

[Mutes]

Depending on what OS their phone is it's possible. Not just with a phone number though.