Configure your Yahoo account on tour iPhone and enable contact syncing.
1. On your iPhone, go to Settings - Mail, Contacts, Calendars, and then select "Add Account".
2.Select "Yahoo".
3. Enter your name, yahoo email address, your Yahoo password and a description of the account in the appropriate section, and then select "Next" in the upper right corner.
4. When the account has been configured, slide the lever so that green appears (enabled) next to "Contacts" and anything else associated with your Yahoo account that you want synced to your iPhone.