Two things come to mind: 1. Have you been tinkering around with your programs, in their options? Perhaps you set something to check for updates automatically at startup? Some will try to dial out if the connection isn't already there.
2. Could also be you got some spyware landed on the computer, maybe from a web page? Try a spyware scan with adaware, available free from www.lavasoftusa.com - spyware is a small program tries phoning home, which is one thing can force a dialup attempt so persistently.
safeTsurfa Stay alert! Stay safe! Stay alive!