Using the HP L7580 for faxes and phone calls on a single line

I've been using the HP L7580 scanner/printer/fax on a dedicated fax
line and it works fine. However, I've now moved it onto the same line
that I use for my phone as I get so few faxes. The instructions
suggest that this isn't a problem and that I can set it up so that it
accepts faxes in my absence and leaves voice calls alone so that they
go through to my answerphone. Trouble is that I can't get it to work!

I have even rung HP with no luck. Have I mis-read the instructions or
is it possible?

Would appreciate any help.

Peter