How to setup your iPhone with ActiveSync

To configure Exchange access on your iPhone

1. Open Settings -> Mail, Contacts, Calendars
2. Add Account -> Microsoft Exchange
3. Enter your e-mail address for e-mail (Ex. user@domain.com)
4. Enter your e-mail address for your username
5. Enter your password
6. Click Next. If you get a certificate error click Accept
7. Enter "owa.exchangecarrier.net" as Server. Click Next -> Save.

Additional help is available via Apple's website here and here.