Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
What 'enter bill code' is, depends I think, either upon the number dialed (e.g. if you dialed a 'call card' number, entering a bill code would be needed)... or else it depends upon the phone system, or stuff outside the fax/the HP device.
Also a good question I have to ask is: If it states, "Enter bill code", is that an audible statement, a sound on the line in other words, or does it appear on the HP machine's display itself ?
If it's a sound on the line stating to enter bill code, then you have dialed into some sort of number that requires such a billing code, you should re-examine the fax number and also dial the voice phone # ***** with it for an inquiry as to what billing code is needed to fax into that number.
If it is a message on the HP machine's own display... then at this point I need confirmation of just that fact, for it is beyond my experience, though not necessarily beyond my researched resources, to say what that would mean.
Let me know which of the two it is - or, if I have already answered your question adequately, please rate my Answer. Thanks.