By default, contacts, calendars, and other information is automatically synced when you connect your iPhone to your computer. You can prevent a specific iPhone from syncing automatically by connecting the iPhone, selecting it in iTunes, and deselecting â€œAutomatically sync when this iPhone is connectedâ€ in the Summary pane.
You can also prevent automatic syncing of all iPhones.
To prevent automatic iPhone syncing:
In iTunes, choose EditÂ > Preferences.
Deselect â€œDisable automatic syncing for all iPhones.â€
If this checkbox is selected, the iPhone won’t sync automatically, even if â€œAutomatically syncâ€ is selected in the Summary pane. To sync your iPhone and your computer, connect the iPhone and choose FileÂ > SyncÂ iPhone. You can sync iPhone with only one iTunes library at a time.
For more information, see the iPhone user’s guide or the Apple support website.