I am helping a friend to re-arrange his office and trying to avoid
having to run extra CAT5 cables.

I have been looking at the Hellerman Coolport range of products

http://www.rwdata.co.uk/index_news01.htm

which utilise unused pairs in the CAT5 cables to either.....

Get 2 Data down 1 cable run

or

3 Voice (Digital)

or
1 Data and 1 voice (Analogue)

Or

1 Data and 2 Voice (Digital).

So, to the question(s)

Firstly, are all digital phones created equal? Will even the ones with
lots of buttons and LCD displays work with just 1 pair?

Secondly, Will an analogue fax machine which is being run off a PABX
work with just 1 pair (4&5) or will it also need 1&2 (There is no R
button, but it would be nice if it rang).

Also, another requirement is to "extend a bog standard BT analogue line
from nearby the patch panel to nearby one of the CAT5 outlets. Can
anyone give me catalogue details or point me to a good site where I can
work out what is required at either end?

Thanks