Please be advised that having been *spammed* too many times by said ISP's 'anti-spam' software, because my address was *forged* as the sender of messages being 'challenged', *ALL* such 'challenges' are blocked at time of transmission.
You ask for a favor, and then expect people to jump through hoops to tell you about it. "Sorry, Charlie", that dog doesn't hunt.
As for your 'problem', the real solution is "training". It is a 'personnel' problem, not a 'technical' one.
Technology cannot _prevent_ the problem, although it can "ameliorate" it. a DTMF decoder chip, wired to interrupt the PA leads when tones are detected is about as close as you can come. Put the decoder _in_front_ of a 50-100ms 'delay line', and you can cut the PA off _before_ the tones propagate through the delay line.