Cannot join WiFi network?

You haven't given us much to go on.

Can you connect to other wireless networks from your phone?
Are you getting any error messages?
What kind of security does the office network have (WEP, WPA, WPA2, WPA Enterprise)?
Is it a "classic" or a 3G phone?
What version of firmware is on your phone?

The two most common reasons you can't connect to a network are an incorrect passphrase and a router with out of date firmware. The fact that another iPhone connects makes the out-of-date firmware an unlikely cause. Next is a router that is set to only recognize specific devices, called MAC filtering (MAC is Media Access Control, not a computer type). If the router does this the WiFi address of your phone (settings/general/about) must be added to the router "allow" table by your admin.

To test the first reason go to Settings/WiFi, tap on the arrow to the right of the network SSID and tap "Forget this network". Then try to connect again and enter the passphrase very carefully.

Apple also has this helpful tip for resolving WiFi problems: http://support.apple.com/kb/TS1398?viewlocale=en_US

You are sure the other iPhone works on the same network? I didn't think that WPA2 Enterprise was supported. At least it wasn't in version 1.1.4.

If it works on other networks then you have me stumped.

Did you check the network settings as described in that tip? what is the assigned IP address?