I have found a few yaers back that in Winnt 4 that many internal modems (even rated for nt4) did not work wll in the OS. Since then our company has only recommended External Modems to all customers.
Since our services are contracted out, when a modem fails, we only supported externals (although we would try to get the others to work). Externals releive so many problems and the price is definately worth it. I will never go back.