Ah, ok, I misunderstood- I thought you were connecting to the VPN on the same router you were sitting behind.
It's possible your firewall at work is blocking ESP or doing something else that will break ISAKMP from the iPhone.
Make sure on your firewall at home you have NAT Translation forced on for IPsec, and if you can check on the work firewall, make sure it allows you to use udp/4500 outbound as well as udp/500 and esp if it can't do NAT-T for some reason.
If you are at a remote location and it works from 3G but not their wireless, there may not be anything you can do to fix their wireless if they're blocking it, especially if that blocking is done on purpose to prevent exactly what you're attempting.
You could always jailbreak and run OpenVPN on a UDP or TCP port they allow out, but depending on what they pass/block through your work firewall that may or may not work either.