- Do you understand what is meant by "ATM machine" and "PIN number"?
- Is there any possible ambiguity?
- Does it cost you anything to hear "ATM" and "PIN" used incorrectly?

If the answers are as I suspect, then what does it matter?

I always use "cash machine" rather than ATM and would have to spend a second or two figuring out what was wanted if ATM was used.

As "PIN" is pronounced as a single word, a possible, though admittedly unlikely, ambiguity could arise relating to pins used in other contexts. I think "PIN number" is an improvement.

DC