modem problem

I have win xp pro and I just installed xp plus, now it seems that when i try to dial out with pc anywhere it will dial out fine the first time, but if I try to dial out again it says that it can’t find a dial tone. If I unplug the phone line and then plug it back in then it will dial out again but only one time. I have a US Robotics 56k modem (internal). any ideas?