Hello plum10021,
Welcome to the HP Support Forums!
I see you are interested in setting up the fax on your HP Officejet 6700. After taking a look in your 6700 Specifications in order to have a working fax you would need to have one of the following:

Please follow the link to get started: Setting up the HP All-in-One to Send and Receive Faxes, once you are all set up to send and receive faxes take a look below:
Send a Fax from the Scanner Glass or the Automatic Document Feeder
I see that you do have the ability to use eFax, it is service provided by j2 Global Communications, Inc., not Hewlett-Packard. It allows you to send and receive faxes using the Internet instead of a phone line. eFax is easy to use, but the service differs from a typical phone-based fax machine. You cannot send eFaxes unless your printer supports eFax. It is not a free service, but it gives you another option, other than getting a new phone line or sharing the one you currently have.
I hope this answers your question, if you need any more help please let me know.
I found this information here: HP Web-Connected Printers and Supported Cloud Services
Thanks,
HevnLgh
I work on behalf of HP
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