If you are thinking of putting a ad in Yellow Pages DONT PUT YOUR FAX NUMBER IN THE AD
WHY
Because you get faxes sent, trying to sell you all-sorts, and I got woken at 3-30am this morning by one 
Andy
I believe that has been illegal for some years now - not least because they are using your toner and your paper. Whether you feel it's worth reporting to a relevant regulatory body is up to you though Andy.
I have a tip for anyone (including yourself) who DOES want to include their fax number in their advertising without being woken up and without increasing their toner bill. For some years, I used a fax-to-email service. The fax would be an email attachment (picture file) that was forwarded to you from the hosting companies server. I used to pay about a tenner a year hosting fee for the virtual fax number. I think there was a small charge to send faxes but I only ever wanted it for receiving so that wasn't relevant for me.
As a picture file, the fax could be manipulated in picture viewing software - useful if the sending company got their file out of scale.
This was the company that supplied the service (looks like they charge more now as well)
http://www.nildram.net/fax_services/Looks like £20 p.a. and £10 setup fee.
I imagine there are plenty of other companies that supply this service too.