HP Photosmart C410 fax is connecting to home phone line instead of dedicated fax number

I have a Photosmart C410a.  The machine should be connected to the dedicated fax line, and this line's number has been input into the fax header.  However, when tested using the the FaxBack Service, the return fax calls the house line instead of connected to the fax line.  The machine is connected directly to the wall and there's no splitter, etc, with the house phone.  The home phone number has not be typed into the C410a at all.  Any thoughts that you might have about what could be going on would be greatly appreciated.

Best,

Scott