Hi Shaun,
Can you give us all the connection details, like MDI, Bluetooth fitted etc etc.
It sounds like you might need to swap 2 pins around on the connector block. If you compare the pin allocation diagrams on rcd300 & rcd510, the cd player ones changed, not 100% sure...maybe....someone will come a long to advise on that.
Yeah the phone button on mine just mutes. You will need a Bluetooth interface as its not built into the rcd510, the vw one is silly money. I've got a parrot kit fitted to mine for example....