First one needs to connect their iPhone to a computer with a USB cable, double or right click on the iPhone icon located on the source panel of iTunes to select by clicking back up, click on the eject button on one device and remove the docking cable, insert the iPhone to be able to the backed up information on the other device. The other way on how to transfer information from one iPhone to another is by syncing the iPhone with iTunes for people who have information saved in other locations like the yahoo address book, at yahoo click options then mail options and finally contact option and then the import/ export feature. Sync this to your iPhone using iTunes and check on the sync option to sync it with the address book, if using a Mac one needs to address book importer to get a CSV into it and sync using iTunes.
One could also transfer their contact information manually although tedious this methods ensures that the information through to the other device, and once transferred to the device one could sync them into another phonebook and can always use the backup and not have to transfer contacts every now and then.