Usually this message occurs when your dial-up modem or another device is using the phone line at the same time. The easiest way to check this is to disconnect other phones and devices that may be using the same phone line, and see if the message goes away.
If this does not solve your problem, disconnect and reconnect the power cord.
If the problem still persists, the line interface Product (LIU), the part that the phone line plugs into, may be defective or the HP product may need to be serviced.