change keyboard to phonepad?

Only for that one piece of software. No app is allowed to change any aspect of the iPhones core OS, so if you did find a T9 app (the technical name for the phone pad) it would not work in Mail, Messaging, Notes, etc.

Is this what you want?

http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=319345350&mt=8

No, you cannot. You can send feedback to Apple that you would like a T-9 keyboard - http://apple.com/feedback

You can't change it to a phone pad. You can get access to the numbers by hitting the 123 button on the lower left of the keyboard but there is no option to turn it into a standard telephone keypad.

Some applications have it such as Youmail or the Phone, but it has to be built into an application to do that.