Iphone Config to Exchange server 2003

Hello,
I am running Windows 2003 with exchange server 2003 service2.
I can check email using outlook web access typing Ipaddress/exchange.
I tested oma same thing with IPaddress/oma.
I can get the screen but when i try to do with Iphone I cannot able to connect to exchnge server.
It comes with the error messag
” Exchange account verifcation failed”
Am I missing any thing to configure,
help me out