Hi again, thanks for the tips everyone.
A bit of an update - This is what it looks like at the moment:



The split polygon tool seemed to work really well once I started with half a sphere instead of trying to split the main shape. I extruded the edge of the sphere out a bit and split around the buttons, then cut the same shape out of the main phone shape and attached the two together. The other buttons were split and extruded. I'm a bit concerned that I've done this in the wrong order now as I'm having trouble smoothing the edges with the smooth polygon tool without screwing up all the finer details. Should I have done this first or is there a way to smooth out particular bits?