How to correct iPhone keypad mistyping after reaching primary number

When you call a business number, there is often an internal phone tree to be negotiated. After reaching the main number, I find it all too easy to mistype a number after I have already delved down two or three layers into the phone tree. The backspace key on the iPhone keypad disappears once you reach the main number. Is there anyway to make a correction to an entry mistyped into the phone tree? The only way I have found to handle this problem is to start over from scratch. (Sigh!)