PPTP Problem with iPhone4

Hey guys,

i hope you can help me out with this.
i have a freshly installed UTM9. Everything is running fine so far, but i have one problem.
I activated PPT. With my Android no problems at all.
With my iPhone 4 i get only one error in the log when trying to connect:
MGR: dropped small initial connection

When connecting from my WLAN, i get an IP from the PPTP-pool and everything works as supposed.

Could this be a carrier-problem?

Hope you can help me out.

Best regards
Dan