I would like to know, what limitations there are for how far one can go in terms of replacing UI components of current touch screen smart phones, in particular iPhone, Blackberry and android based phones.

What I would like to do is create a custom UI for dialing out and incoming calls.

I have some experience with Blackberry development. The theme builder for it, can be used to customize certain items on the incoming call screen, but it doesn't look like that you can increase the size of answer button. I know Blackberry also gives you access to all the phone APIs, but I'm not sure that you can create your own UI that can gain preference over the Blackberry incoming call screen. And if you try to customize the incoming call screen by adding any buttons to it, they would be rendered as pictures.

I could possibly design a complete UI for android, since different manufactures have different UI for android based phones.

Can I do what I want to do using iPhone, Blackberry or android? Or any other phone for that matter? I am guessing may be for Nokia phones using Qt, but I prefer the 3 platforms I listed.

Thanks for all your help.