Are you looking for something like
this[
^]? Note that I don't know anything about it, this was simply the first link returned by a google search for "c# modem library".
Adding... I failed to mention that the same google search also yielded results that appeared geared toward using a GSM phone as a modem, so if that's what you're looking for, do the google search I specified and take a look at some of the links.