My husband and I both have the iPhone 4s, he has had his for over 2 years, and I recently activated one I received from a friend. I did a complete reset of the phone before I installed my SIM card. We do not share an Apple ID, and neither of us use the cloud. All of the iCloud setting are in the off position on my newly activated phone. His phone wasn't even in the house when I activated my iPhone. We even use separate computers for our own individual phones and separate iTunes accounts. Can anyone explain why this happened? What happens if I start deleting contacts on my newly activated iPhone that somehow came from his phone, will they also be deleted from his phone? How can I keep the phones from syncing in the future? We don't need each others contacts to continuously sync on our phones. Please advise, thank you.
A few possibilities:
Somewhere down the line, one of you accidentally combined and synced your contacts.
You added a non-iCloud account in "Mail, Contacts, and Calendar" that contains the contacts unknowingly.
You restored from a backup during setup that contained the contacts.