I have just been setting up L2TP with iPhone connectivity. The following may help.
L2TP uses 3 ports. UDP Port 500, UDP port 1701 & UDP port 4500. You will have to forward these 3 ports to your VPN server.
I found eventually, that the iphone has a problem if some non alphanumeric characters are used in the Shared Secret. It doesn't even attempt to go over the network. I changed mine to all alpa numeric (Mixture of upper and lower case is fine) then it worked a treat.
Hope this helps.