Having actually done this way back in the mid 90's, I think the OP wants to send text messages. So, I remember next to nothing other than it was possible, and it worked, and <rant> pagers back then were a whole lot more reliable than the damned SMS message delivery for cell phones. (It once, fairly recently, took 3 hours for a message to arrive on my phone after it was delivered. I was on call, so that message that could have been delivered around 10:30pm, instead arrived sortly before 2 am. My boss had been called because I was "unresponsive".)</rant>

Ok, so, I remember that there was a name for the protocol to send text to pagers, but don't remember the name. And I seem to remember you called a generic service that given the message and the pager's phone number would forward that message to your pager's service. That about covers what I remember.

So, some obvious questions, what are you calling when you dial via the modem? Are you attaching to some other ISP? Then you should be able to just use the networking stuff you've found...

-Scott