Combine address (phone) books
 from different sources

You can also use your device to combine different address books in iTunes, such as Outlook on a PC and Address Book on a Mac. Doing this will load all of your contacts onto all of your machines—your iPhone, PC, and Mac. Here’s how you do it:

1. Sync the device with the first computer’s contacts.
2. Then sync it with the second computer by choosing a different account in the iTunes Info tab (such as Outlook) and clicking Apply.
3. iTunes will then ask if you want to “Merge Info” or “Replace Info”—choose “Merge Info.” All of your iPhone’s existing contacts will remain, and the second computer’s contacts will also be added.

Repeat the process with other computers.