do smart phone companies like htc and palm make their own boards for their devices?

[trippy24]

i was wondering if smart phone companies use some sort of standard boards for their devices or do they have to make their own custom ones for every new device they make? and if i got a hold of one and some other parts, could i essentially build my own phone from them? i have the ability to prototype my own case for it and i want to attempt to throw in a few other functions (extra buttons) if possible