Actually, you may get rid of most of the hiss by turning the volume down 5 degrees, just turn it down '2 hairs'. Try it, you'll be surprised. If that doesn't help, see if you can clear the hiss by shorting the t and r. If so, you know what to do next. That's right, a set of relay contacts that short t and r when not paging. Often times there is a page relay in the phone system. Usually it is N/O so you'd have to use it to control another relay, but you've got 24V right there. And the page level is low, so you don't need big honker contacts.

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.