Dial-up / Modem problem...

right. basically, for some reason all of a sudden, my ibook won't connect to an ISP. I have tried three different ISPs (each of which work on other machines) but with each one, it dials up, then the remote access status panel says "Connected at unknown rate", and then says "No carrier Signal". It was working just a few days ago, so I really can't understand what's gone wrong

thanks for your help in advance,
Pete