I'm confused? It works perfectly if you wait for the phone to ring, then answer it, then put the phone down, then type in numbers. The bomb screen is disabled while the phone is visible, so that is not the problem. I have no doubt there is bugs, but It works fine for everybody that play tested it :/
Edit: I see your problem here, you need to punch in the numbers when the phone is down, then once a number is put in, you pick up the phone.