iPhone reception outdoing home network

Ok so I think this is the right place to ask this.

When I am at home, my iPhone 5 chooses the phone reception over the home network, so when I download music it is coming out of my data usage on my phone bill instead of my home internet, is there any way to stop this?

I have the latest AirPort Extreme on one side of my house, and the latest AirPort Express bridged wireless on the other, my house isn't too big it might be maybe max 20 meters across, my phone and both AirPorts are all running the latest software as well.

Thanks for any help