There is another HUGE advantage to using an external sound card for software modem functions. The majority of the operating system software is unaware of it's existence and will not try to use it for some default function thus downing the software modem's connection to the interface.
Having a back up headset jack splitter is still a good idea though. 2 is indeed 1 and 1 really is None. That instructive mantra is meant to impress upon the uninitiated that having 2 ways of doing each critical task means that a failure in a single piece of equipment should not cause a functional failure.
--
Tom Horne W3TDH
Tom Horne