He might want to be using something that has an LCD display so that it can display the caller ID of the extension that has called it.
In fact, he might want a group of them in case there are multiple panics at the same time, otherwise the second panic won't get through. The more I think about it the more I think this isn't something you should be doing with your phone system.
I know in some government offices/healthcare facilities they have panic buttons, but they aren't part of the telephony system, they are part of the security setup, so I've never really paid much attention to how they use them.
Having said that, I was once in a facility when the panic was triggered and it locked everything, I was stuck between two gates and security were communicating using 2-way radio and intercoms dotted around the place. I had to spend quite a while pleading to be let out....So not really a phone thing at all in that case.