Stop syncing iPhone 6 & 7 call history

I have an iPhone 6 (iOS 10.3.2) from the company I work for, which I also used privately. Since one week I have a private iPhone 7 (iOS 10.3.2) to keep things seperated a bit. I installed the latest iCloud copy of the 6 onto the 7, because I still wanted to have most apps, my music, photos and contacts from the 6.

Now my call history from both phones keep synchronizing. When I make a call or receive a (missed) call with the 6, it also appears in the call history of the 7. And vice versa. How can I disable this?

Thanks in advance for your help!