How to enter numbers during a call on IPhone?

I'm trying to use a top-up voucher on my iPhone, but when I get to the part where I have to dial '1' to move on to the next step, it doesn't work. I don't know how to enter it.
Help, please?

Added (1). I've got to the bit where I open the key pad. When I press one, nothing happens!

During the call, there's an option on your screen that says keypad, press it and it will pull up your keypad. Hope this helps.