How to press "Call" or "Send" button while you're talking??

Normally, cell phones have a "Call" or "Send" button outside the screen. So you when you're talking to someone, you'll be able press "Call" button. For example, I called a company and they needed me to press the number xxxxxxxxxxx and press "Send" or "Call" to send the data I just typed from my phone.

But I don't know how to do this with iPhone? Any idea how to do it?

Thank you in advance.