There is no ISA client for the iPhone.
An option is to create a rule for the iPhone that says that it cans urf without authentication. You could use it's MAC address to create a DHCP reservation for it and then base the rule on the source IP. It is not very secure but would allow you to use the phone.
The other options is to enable the ISA to prompt for creditionals when it finds the device is not identified... But as to how to do all of this! you need to post a question in the ISA Zone, there are a numner of great experts there that can sort you out...