A fax machine or fax modem will plug into any phone line in which a
regular telephone will work. You pay for faxes at the same rate you
pay for voice calls. Since you are in the United Kingdom, you know
more about the rates than I do. The fax number is whatever number
is associated with the phone line to which you plug in the modem.
It is only important if you expect to receive faxes. It is much
easier to use the computer to send the occasional fax rather than to
have it receive all incoming faxes. Faxes are image files and take
a lot of storage space on your hard drive.
You can only fax a document which is already stored on your
computer. If you created a letter with Microsoft Word or Corel
WordPerfect, you would print to the fax to send the document. You
will be asked for the telephone number to which you are sending the
fax. Make sure it is a number to which a fax machine or fax modem
is attached. If your recipient shares the same telephone number for
faxes and voice, you may have to call the person to give them a
heads up that a fax is on the way. If you have a paper document to
fax from your computer, you will have to scan it or take a digital
photograph of it to get it on your hard drive before you can fax it.