iPhone fallback to 3G

First my apologies if this is not strictly a programming question but maybe the solution requires some programming.

I have a WIFI hotspot (RaspPi) which serves files to my iPhone but does not provide internet access. If my iPhone is connected to this hotspot then all internet requests (email etc) from my iPhone fail. The iPhone needs to send all requests via the 3G connection despite having a WIFI connection. How do I do this?

Any pointers to a solution at this stage would be appreciated.