How to generate a verification code/number?

Question

I\'m working on an application where users have to make a call and type a verification number with the keypad of their phone.

I would like to be able to detect if th

Answer 1

• I must have a reasonnable number of possible combinations (let's say 1M)
• The code must be as short as possible, to avoid errors from the user

Well, if you want it to have at least one million combinations, then you need at least six digits. Is that short enough?