Don't own the sound card, can't help there, but something doesn't seem correct.
Buy the phone coupler... There is a lot more going on in there than just providing audio in and out. In order to keep the feedback and other annoying interactions down, you need to be able to separate the caller from the return mix, this is one thing that a good hybrid will do. Consider buying a better analog hybrid on ebay, they can be had for under $100 and will work and sound better than a $50 new interface.
If you simply want to couple the phone to a line input, all you needed was a cheap little green 600:600 ohm transformer from Radio Shack, and a .1 microfarad capacitor. Connect one side directly to the phone line pair (side with the cap to block the DC), the other side to a line input on the mixer. Will probably work as well as a headset connection.
+ - _____________
phone line |transformer | line input
Sorry, best I can do with text.
The || is the capacitor that is used to block the DC so that it doesn't saturate the transformer, needs to be over 100 volts or it could pop. Remove the telephone handset while using this because all audio goes through the line and the handset can be distracting. This device is really only good for one way audio, for two way audio you really need a hybrid or at least speaker phone so that one side gets cut off while the other is sending audio.