Modem cannot get dialtone

Hi,
I have a box running XP Home. The modem and AOL where causing issues with the phone line. I could not get it to disconnect after a dial attempt. It would never dial in and connect to AOL. I have flattened the box, reinstalled AOL tried 2 different modems, confirmed the line has a dial tone and still cannot find dial tone. I tried connecting to another ISP same results. The modem is installed in device Manager and it is using IRQ 21.
Any thoughts
Thanks