a poster here http://www.amazon.com/D-Link-Skype-USB-Phone-Adapter/product-reviews/B000E39V9O/ref=sr_1_1_cm_cr_acr_txt?ie=UTF8&showViewpoints=1
suggested using the USRobotics driver and another says the D-link Vista driver is better than the one on the DPH 50-U CD.
I installed the USRobotics driver and it works. For details see my other post
http://forums.dlink.com/index.php?topic=7114.msg42927#msg42927