USR 56k Modem does lock Dial Up Prog

Why does my 3COM/U.S. Robotics Modem (White, made in Romania) lock up the Windows provided Dial-Up prog (this happened while using 2k Pro & XP PRO), so that I have to wait about 1 – 2 minutes to re-dial to my ISP. This never occurs when I use a Linux distribution. So why does it lock up the Dial-Up prog ?