How do I use I2C from an Arduino Uno when I have a GSM shield mounted? This may be an extremely obvious question but I don't seem able to find a definitive answer.

I have an Arduino Uno and a Sim900 GPRS/GSM shield fitted on top of it. I had I2C working on the Arduino to communicate with a Raspberry Pi, but now that I have put the shield on top I can't get the Pi to detect the Arduino. As far as I can tell the SDA and SCL pins on the Arduino aren't being used by the shield so does anyone have any idea how I go about this?

Thanks
There seems to be a number of different Sim900 shields from different suppliers? I suggest you check/post the datasheet of the one you're using. If I2C works without the shield, and does not work with the shield connected , then it seems logical that there is a pin clash.
There is a good Random Nerd Tutorial on getting started with the shield on a Uno. Maybe test it out completely before connecting to the Raspberry Pi?
The Raspberry Pi is 3.3v and the Uno is 5v. You didn't mention level shifting in your question, but it may be worthwhile.