DECserver 200 is circa 1990 gear.
You're probably going to have to rewrite (some of) this LAT software stuff. A partial rewrite is probably more palatable than a complete replacement, but there are commercial software tools and packages around that send out pages, and probably some open source for this purpose. And even the rewrite itself likely isn't hairy.
This because the existing software almost certainly doesn't deal with dial tone and dialing out -- and if you don't have the leased line, you're going to have to add code to do that. (There are a couple of ways to implement this, too, whether using language I/O or $qio[w] or LAT-specific calls.)
That, or figure out how to have the modem do this automatic ringdown, or to create an outboard "leased line emulator" box to deal with this, and that approach probably just as much work to deal with -- and more likely to tip over.
Or you get the leased line, such as what's sometimes known as a ringdown line; a dedicated line.
Sorry.
On the plus side, reading off a database and sending email to the paging network or connecting more directly into a paging network or such is straightforward programming. If you're sending numeric pages, those can usually be sent from a touch-tone dial sequence, and that's easy and it requires minimal hardware.
I won't suggest Asterisk or VoIP, but that'll do this sort of stuff, too.
