When I changed jobs, I found myself with a lot of legacy contacts that were on my iPhone but not in Outlook at work. If you find yourself in a similar situation, you can use an app called Orbi Tools to move contacts from the “On my iPhone” group to Exchange. I did this, and it worked perfectly, although you should be aware that any photos associated with your contacts on your phone will likely not make the move to Exchange, because of how Outlook stores image information. Also, if you have a few hundred contacts or more, you may want to plug in your phone before starting the move — it can take quite a while to move all of the contacts, depending on the speeds of your internet connection, your Exchange server, and other factors.
Hat tip: The Sysadmins