[nradov]

It's a nice idea but wouldn't work in practice. If the "help" PIN is always 1 higher then there would be too many false alarms due to pressing the wrong button. Or if the "help" PIN is totally different then victims will forget under stress.

[jmonegro]

Also, robbers would know there is such thing as a "help" pin, therefore rendering it useless. It's not a good idea in practice.

[Angostura]

It wouldn't render it useless if the behaviour of the machine was identical (other that sending a silent alert) and still dispensed the cash.