I am getting an interrupted dial tone when trying to access my internet dial up.?

i have checked the settings on the computer and the modem says it is working ok. bt have checked the line and that says it is ok, but whenever i try to connect the link is dropped before the number is completed and a voice message says that i should check the number and try again. the number came with the install disc. it is an intermittent fault but more often than not we are unable to make a connection to the internet. does anyone have any ideas please? thanks