You should have 8008 and 8443 both open on the firewall, first off. I'm assuming you're not using the NAT and Firewall services in OS X since you're connected to a firewall.
If both ports ARE indeed open, I suggest that you verify that the certificate is properly installed on the iphone or device that's trying to connect.
If it is, and you're sure that all the settings are right on the remote device, then the server is probably to blame.
I'd start my trouble shooting by seeing if the server will work when NOT using a secure method.
In Server Admin, select iCal. Select Settings and click the Authentication button.
Under "SSL" choose Don't Use.
Save your settings. If it doesn't ask you to restart the iCal service, you should do so. (most likely it will)
Now, on your remote devices, create a new Calendar connection to the server that doesn't use SSL.
If it connects, it's a firewall problem. (again, assuming your certificate is valid)
If it doesn't connect, it's most likely a remote device problem.
-G