I always come from the theory that a telephone number should be visible on every page of you website (unless your business strategy doesn’t involve people contacting you:-)
A web norm is to have the number on the top right hand corner of the site, and it makes me glow whenever I see one there.
By having a telephone number clearly visible I know without thinking how to make contact. On a website that requires a process (for example an e-commerce purchasing system or online banking process), the telephone number should be visible in case a visitor encounters errors or does not know what to do next. If this occurs on an e-commerce site, the visitor is more than likely not to continue. A telephone number could be the way out.
The other day I was on one of our banking sites where we process all our Debit Orders each month. After logging in and trying to process the order I came across a problem and I needed to change some settings. I had no idea how to do this, so I searched for contact details, only to find I had to log out to get to the contact page. Once a visitor has logged in all access to contact details are lost. This frustrated me greatly, although banking sites are lucky because it is too much hassle for us to try and change to a new bank.