Should never have to give out your password over the phone, but for security questions the reason why its often alphanumeric is you will have people entering security questions like "how much did your first car cost" and they enter something in pounds sterling and pence which the Philippines call center phone script reader who deals in dollars all day isn't going to be able to interpret.

Or "what street name did you grow up on" could include some UTF-8 glyphs from Asian immigrants.