how do I use letters on pad when making a phone call?

Hi

Was making a phone call to my Bank recently on my iphone 4s when I was asked to key in my postcode in answer to a security question.

I found this impossible to do from the pad from which I had made the phone call even though it contains the letters of the alphabet. I really

expected it to act like a normal phone handset where if you press the figure 2 it will allow you after the first press to enter an A or an a and

if you press it twice it will alow the entry of a B or a b. How do I get these letters in the phone pad to work on the 4s? Many thanks for your

help.

Regards

Gerald