So you mean like it is on android? #bwahahaha
I haven't honestly seen much of android at all, so I don't know how it handles calls.
Regardless of the context the interface is presented in, if a certain function is not available, the control should be disabled, not hidden. This leads to a consistent experience. Imagine in Apple in their infinite wisdom decided to hide 'Z' key on the keyboard, since the letter is rarely used. This would:
1) Make accessing the Z character annoying.
2) Make the interface inconsistent, not only because of the absence of the Z, but because all of the other keys would need to shift to fill the void.
Maybe an outlandish example but it illustrates the big problem with inconsistency; In both cases, you can't use muscle memory to achieve your goal.