Phillips, 32, has four years and $50 million left on his contract and the team reportedly decided a month ago to see if they can get back value by trading the three-time All-Star.
Phillips hit .261 with 18 home runs, 103 RBI and a .310 on-base percentage this season for the Reds.
He was upset earlier this season when he felt he was lied to regarding his contract status after Joey Votto signed a 10-year extension.
"To this day, I'm still hurt. Well, I don't wanna say hurt," Phillips said. "I'll say scarred. I'm still scarred. It just sucks that it happened. For [Castellini] to sign somebody for $200 million, there must be a new vegetable or fruit coming out that we don't know about. For him to do something like that and tell me they didn't have any more money, that's a lie. But what can I do? I just feel like it was a slap in my face."