My home broadband is with spark UFB, using a fritzbox. I've configured the fritzbox vpn server so I can connect to my home network while I am away. (more secure than port forwarding).

This works just fine using my Skinny iphone , i just enable vpn. 

Except.... when I activate skinny mobile data binge.  Skinny block something so that I cannot connect to my home network via my mobile vpn.  Why would they do this? It is the reason for using data binge , to download some bigger files. 

I'm guessing they block a port (500?), anyway around this? I don't think I can change the port on my iphone though. It defaults to port 500 under ipsec with no option to change it.