Using Keypad whilst on the phone

This is probably very stupid, and I didn't used to have any problem. If I'm answering an automated call, the sort one gets from ones bank, and they ask you to input a four number code, I never get it right anymore. I either end the automated call, because only the red button on the phone is there, or if the green button is there it thinks I'm trying to dial that number. This must be simple surely? I can speak the code obviously, but sometimes they don't understand. What on earth am I doing wrong. Phone when it rings also, sometimes has a band at the top with a small green and red button, other times, larger ones towards bottom of screen. Seems to be no logic or reason.

As I said, probably just being stupid, but I'm getting on, trying to keep up, but the day will come where I'll level off I think. Trouble is it annoys me tremendously, not being able to understand something. Using an iPhone XR.

Thanks in advance if anyone knows what I'm talking about.